MathQuill is a web app for editing formulas in Google's Gmail. It lets you edit formulas by entering them directly into the mail, as well as editing existing formulas. The app can edit math formulas for a variety of fields, including calculus, probability, finance, computer programming, geometry, statistics, and more.

Use it with

Gmail

You can use MathQuill for Gmail to create math formulas and add equations to your mail messages, including replies and comments.

The app is supported by Google's own MathML parser, which understands most math formulas you enter. The only exception is that it does not support math inside URLs. It is also not supported by the MathML Editor in WebKit.

To edit an existing formula, press the mouse button on it. 

MathQuill is free to use. It is not open source, but you can install it on your own server. The app is also completely free.