Mimetex is a web-based program that generates LaTeX images (math) on the fly, if you use it like this: <img src="http://www.forkosh.dreamhost.com/mimetex.cgi?c=\sqrt{a^2+b^2... alt="" border=0 align=middle>.

To do a plugin that does [tex]\sqrt{a^2+b^2}[\tex] is simple (PHP).

   $text = preg_replace('/\[tex\](.*?)\[\/tex\]/ie',
   "'<img src=\"/cgi-bin/mimetex.cgi?'.rawurlencode('$1').'\"          
   align=\"middle\" />'", $text);

Mimetex is great beause A) you don't need to have a full TeX installation on your server, the program generates it internally B) you don't need to worry about scalability - mimetex can be compiled with caching enables, which makes it the same as static images

firefox extension based on mimetex

Pros: - Works on any site (I've only checked it out in gchat though).

Cons: - Not as pretty (if it matters to you) and requires both sides to d/l firefox extension really (not for emails though).

I really like the code-creating buttons and reference page. They make it faster and easier.

The equations seem to be displayed a bit small though. My test paste (http://mathbin.net/4315) doesn't go that deep into fractionage and the like, but it's already a little hard to read some of it.

Then I did a deeply-fractioned 1 (but not so deep as to be ridiculous) and the e is barely legible.