This browser plugin allows you to edit formulas in Gmail messages using a graphical formula editor.
The script uses MathQuill as the formula editor, and the CodeCogs server (with their generous permission [1]) for generating the images.
See the Wiki for more information.
[1] Please note that if you are generating a lot of equations from a single domain, i.e. over 3000/day, then you may need to arrange a commercial licence with CodeCogs to support continued hosting of the service. Please see their terms and conditions: http://www.codecogs.com/latex/usage.php.