I wrote a little program called GmailTeX which adds $\TeX$ capability to Gmail. You can get it here.

GmailTeX now includes the "live" fully automatic mode, just like when you type a question / answer on MO. (Actually, faster than the current MO math preview.)
It
Added 9/1/2010: Thanks to Kristi Tsukida, it is now available as a Firefox greasemonkey script: http://userscripts.org/scripts/show/84589, and as a Google Chrome extension: https://chrome.google.com/extensions/detail/eenhaafocdcfjengcoflbdnoljeaepgb, for super easy installation.
Added 11/24/2010: Now available as a Firefox add-on.


