Update
Following on from Mike Renfro’s comment below recommending A2 Hosting I contacted them to ask about their LaTeX hosting and this is their reply:

“Yes, we either have or can install the LaTeX packages upon request on any particular server. It’s not a problem as long as it does not become a server load issue. We are currently using the CentOS 5 latex package tetex-latex-3.0-33.1.el5 but if anybody wishes to install their own copy of LaTeX locally on their account they can always get the appropriate binary and do that also if they need a different version.

We would certainly be happy to have you point folks in our direction. We will go ahead and add a knowledge base article on the subject as well.”

A. Images

Unfortunately not every host offers LaTeX but there are sites that can help generate the images that can be downloaded.

1. LaTeX Equation Editor. This innovative editor has a symbol table for those who are not sure of the LaTeX code as well as allowing the code to be typed directly. It uses Ajax so that the page does not need to be refreshed to see the rendered image. The source code uses LatexRender. Update: see CodeCogs Equation Editor.
   Hamline University Physics Department Latex Equation Editor is based on the same code, with some innovative additions. Editor Online de Ecuaciones Latex is a Spanish version. Online LaTeX Equation Editor has modified the code to use mimeTeX.
   (thanks to umustbe and thornahawk for the links).
2. Bruno Gonçalves – Latex Rendering also has a symbol table and direct typing. It uses mimeTeX for a very useful instant preview and LatexRender for the final high-quality image. Discussion is at Professional looking equations by rendering LaTeX online.
3. MimeTeX parses a LaTeX maths expression and immediately emits the corresponding gif image such as this
4. Troy Henderson’s LaTeX Previewer makes it clear what is in the preamble when rendering the image.
   MetaPost Previewer allows you to preview and download images made by MetaPost in a number of formats. You can see the log for any errors.
5. MathTran is a new project by the Open University that intends to “provide translation of mathematical content, from TeX to MathML and vice-versa, and to graphics formats, as a web service”. At the moment only Plain TeX (both text and mathematics) can be converted to an image.
   MathTran instant preview is a web-based TeX system, complete with a built-in help. It compiles Plain TeX code in real time. The source is available at mathtran-javascript.
   Enso TeX Anywhere makes use of MathTran to convert TeX to images in some Windows programs.
6. Roger’s Online Equation Editor offers a choice of image formats, background and text colours, resolution, transparency and anti-alias
7. Sitmo LaTeX Equation Editor uses realtime rendering. It is a Google gadget so can be added to your website.
8. Texify uses mimeTeX to generate the mage. It can also be used to generate links such as http://texify.com/$E=mc^2$ to the image in text based systems such as email.
9. mathTeX is as simple to use as mimeTeX but uses LaTeX to generate a higher quality image like this . If you have LaTeX installed you may prefer to install mathTeX as a cgi program on your system. See also mathTeX Helper, Embedding math with replacemath.js and How to Install Latex On Blogger/Blogspot.
10. HotEqn is a AWT-based Java applet to view and display mathematical equations. Subtitled The IMGless Equation Viewer Applet it cannot be used to create images.
11. Tex2Im is a JavaScript equation editor written by Sergej Zerr which is a web interface to tex2im – the latex to pixmap converter on the server and creates a JPEG image.
12. jsTeXrender is a small JavaScript program which will convert LaTeX code inside pre tags to images. This makes it easy to embed mathematics in any HTML page or in PHP programs such as phpBB or WordPress. It is based on CodeCogs and LatexRender. An example page can be found at Online JavaScript TeX/LaTeX equation render and documentation at YourEquations.com.
13. mathurl is a mathematical version of TinyURL.com. It allows you to reference LaTeXed mathematical expressions using a short url. For example, http://mathurl.com/?5v4pjw will show which you can then edit. More details on mathurl’s help page
14. LaTeX for Blogger is a JavaScript add-on for Firefox with Greasemonkey that enables the use of mimeTeX in Blogger posts.
15. MathBin.net allows you to quickly post mathematics or physics problems in a forum for others to view and reply to and discuss. The posts are long-lived but not permanent.
16. LaTeX word count provides a word count for complete LaTeX documents or for code fragments, with a number of options for parts of the document and whether or not to include mathematics.
17. Equations 1.2.1 is an add-on for the email program Thunderbird, which converts LaTeX mathematics into graphics via a Convert button.
18. Quick LaTeX is a Google gadget that can be added to the iGoogle homepage. It uses mimeTeX to produce the image but it would be quite simple to change the code to use mathTeX or CodeCogs.
19. Latex2png converts LaTeX code into various image formats PNG, GIF, EPS, or JPEG. It includes a menu for inserting code.
20. QuickLaTeX.com is a free service which converts LaTeX code to a URL of an image  along with meaningful error messages. More detail, support plus a WordPress plugin can be found at the author’s blog.
21. jsMath is entitled A Method of Including  Mathematics in Web Pages and uses native fonts, which can be resized, rather than using images. It works best  (but not exclusively) with TeX fonts.
    WordPress and jsMath has instructions for using  jsMath in WordPress blogs.
    Math support in Sphinx can use both image rendering and jsMath for its document generator.
    (thanks to Andreas Maier for the links).
22. LaTeX Composer is an experimental add-on for Firefox. It allows you to see a preview image of LaTeX code before copying the code to a LaTeX-enabled site (the image cannot be copied). Strictly speaking, it isn’t an online application as it installs mimeTeX to the Firefox directory but does offer a simple way of using mimeTeX offline. The program is run inside Firefox and is started using a small icon in the status bar.
23. MathJax is an open source, Ajax-based math display solution which can display MathML or TeX code or a     mix of both in the same page. It allows for MathML to be viewed in browsers such as Internet Explorer which don’t have native support and normally require a plug-in. It works with both HTML and XHTML pages. Previews can be found here.
24. Google Chart Tools will also display LaTeX code. For example,  is given by the URL http://chart.apis.google.com/chart?cht=tx&chl=\displaystyle\int_{-\infty}^{\infty}e^{-x^{2}}\;dx=\sqrt{\pi}
Various image properties can also be set.

B. Complete Documents

Complete LaTeX documents can also be compiled online. Here are a few sites I have come across:

1. LaTeX-Online-Compiler will compile LaTeX documents to postscript, PDF or DVI formats and will generate references. There’s a German language version of the page.
2. ScienceSoft.at can compile a document to various image formats as well as PDF. The resolution can be adjusted and there are a number of templates. There’s a Flash applet version. Again there’s a German language version.
3. LaTeX to PDF uses MiKTeX to convert LaTeX documents to PDF and allows uploading of classes, style files and images.
4. TeX on Web converts LaTeX and plain TeX documents to Postscript and PDF. It has Czech language support. The instructions are in Czech but the site is still easy to use by non-Czech speakers.
5. MonkeyTeX allows you to upload, store and convert LaTeX documents to PDF in your own account. This makes it possible to collaborate on documents and you can opt to make your PDF documents public and searchable. You can also upload style, bibtex and other files which you can use when compiling LaTeX documents.
6. LaTeX Lab is an online version giving a full text editor and compiler complete with menus and toolbars.
   

   “On the Live environment an installation of MikTeX provides the LaTeX processes and packages. A simple C# class library provides an API for interfacing with the MikTeX tools (for tasks such as TeX-to-PDF conversion) as well as the LaTexLab application database which stores users and corresponding file systems. The C# class library is in turn exposed to the Web as an ASP.NET web service which is consumed via AJAX from the LaTexLab application.”

The project is very much under development and the project site is at Google Code.

7. ScribTeX is a free online collaborative LaTeX editor.
   

   “ScribTeX allows you to work on LaTeX documents from anywhere with internet access and share them with your friends and colleagues easily. Some of the many features of ScribTeX include:

   • Create and edit LaTeX documents and automatically render them to PDFs;
   • Full revision control of all your documents;
   • The choice to keep your documents private, allow people of your choosing to view or edit them, or publish them to world. A fine grained permissions system allows for flexible access control.”

8. Verbosus is an online LaTeX editor which can
   

   Create and manage your latex projects and generate .pdf files online, directly in your browser, with syntax highlighting.

Registration is required and the service is free for ‘small projects’ using a maximum of 4 resources, though I am uncertain as to the definition of resources.

C. Other sites

1. LaTeX word count uses a Perl script to count text words in a LaTeX document and has some options to control the count. I’m not sure how useful a word count is for a typical LaTeX document, but this is an easy way to do so.
2. WordPress.com offers free hosted blogs with LaTeX facilities similar to LatexRender. See Can I put Math or Equations in my Posts? for details.
3. LaTeX Symbols Converter will convert accented characters and HTML and XML characters into LaTeX code. For example,
   Schr\"{o}dinger \& his cat
Schr\ödinger \&\#38; his cat
   will both be converted to Schr\"{o}dinger \& his cat.
4. Detexify tries to work out the LaTeX code for any symbol that is mouse-drawn in a box. Thus drawing  should give a list of symbols including \sum, \Sigma and \Upsigma. The mode or possible packages required are also listed. The drawing box is not available in Internet Explorer.

Please let me know about similar sites that are worth including here.

• Asymptote plugin for WordPress
• Asymptote site
• Asymptote introduction and tutorial

-transparent "#FFFFFF"

so

// imagemagick convert ps to image and trim picture
$command = $this->_convert_path." -density ".$this->_formula_density." -trim -transparent "#FFFFFF" ".$this->_tmp_filename.".ps ".$this->_tmp_filename.".".$this->_image_format;

becomes

// imagemagick convert ps to image and trim picture
$command = $this->_convert_path." -density ".$this->_formula_density." -trim ".$this->_tmp_filename.".ps ".$this->_tmp_filename.".".$this->_image_format;

Thanks to Kevin Knuth for pointing out this workaround.

instead of

This is because the ampersand has been converted to the equivalent HTML code.

You can solve this problem by changing one line in latex.php. In
// $latex_formula = str_replace("& #38;","&",$latex_formula);
remove the comment // and change 38 to 038 so that the line reads
$latex_formula = str_replace("& #038;","&",$latex_formula);
Please note: In order to show the code here I have had to introduce a space between & and #. That space should not be there in latex.php

Other odd effects can be dealt with similarly.
If you see a8217; instead of a’ then either
1. use ^\prime in your LaTeX code and/or
2. add the line (removing the space between & and #)
$latex_formula = str_replace("& #8217;","'",$latex_formula);
in the same place as the previous one in latex.php

Both alterations appear in the latest download.

Note: If you want to regenerate older images then you will either have to remove them from the cache or change the code slightly eg by putting in an extra space which will ignore

You can find a whole lot more newcommands at Birmingham (UK) University Maths Postgrad Committee’s Latex Commands page

1. Russian Babel has the \cosec command; so who else uses cosec rather than csc?

Hover over, or click on a formula, to see the underlying latex code.