<a href="http://sourceforge.net/projects/pidgin-latex/" rel="nofollow">http://sourceforge.net/projects/pidgin-latex/</a><p>Adium used to have a plugin for this too, which I used to use, but I haven't bothered to see if there's a current version.
You can also use <a href="http://thewe.net/tex/" rel="nofollow">http://thewe.net/tex/</a> to type out equations on gmail (as well as in the gmail chat).
Note that on <a href="http://www.mathim.com/translate/" rel="nofollow">http://www.mathim.com/translate/</a> when you type anything with a slash in it (eg: "x/2") it tries to go to <a href="http://www.mathim.com/translate/x/2" rel="nofollow">http://www.mathim.com/translate/x/2</a> and fails, presumably because it's trying to interpret the slash as a directory separator.<p>Edit: cool, looks like you fixed the <a href="http://www.mathim.com/translate/x/2" rel="nofollow">http://www.mathim.com/translate/x/2</a> as I spoke. Though the image at <a href="http://render.mathim.com/x/2" rel="nofollow">http://render.mathim.com/x/2</a> is still broken.
I did this a few years back for an independent study in college. Once the semester was over I never touched it again though. Hopefully they'll do a better job.<p><a href="http://carnap.cs-i.brandeis.edu/~dlaz/dev/groupmath/chat.xml" rel="nofollow">http://carnap.cs-i.brandeis.edu/~dlaz/dev/groupmath/chat.xml</a>, for anyone who's interested.
The interface for Mathematica, which typically consists of bracketing special characters with escape characters, is a lot less cumbersome than LaTeX for writing something quickly. For example, the integral of the square root of x is entered<p>esc int esc ctrl-2 x left arrow esc dd esc x. Shift-enter evaluates the thing to 2/3 x^3/2.