Gmail + LaTeX = GmailTeX.
Use LaTeX (a method for typesetting mathematical formulas) with Gmail and Gmail Chat.
WYSIWYG (what you see is what you get) editing of outgoing messages. The other person does not have to have GmailTeX installed or use Gmail in order to view the math. Math is viewable in most mail readers and browsers, including mobile ones.
*** If you are composing using new Compose/Reply, make sure you are NOT in "Plain text mode"! ***
* New in v.5.11: support for theorem-like environments *
** Starting Dec 3, Gmail started serving images through a proxy (googleusercontent.com). This broke LaTeX images which are served from latex.codecogs.com.
Starting with v. 5.9, the new links should work with Google's proxy. To see the old images, you can press F8. People without GmailTeX installed can still read the old math emails in mail readers other than Gmail **