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 TeX for Gmail 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"! ***