NOTE: OS X Mavericks has nearly killed this plugin. It still works in a few apps but will cause graphical glitches in most others.
Inverter is a SIMBL plugin that adds an option to invert the colors of the key window. After installing the plugin, any application compatible with SIMBL plugins will have an "Invert" mnu item added to its Window menu. Activating it will apply a the Core Image filter to the current key window.
After building the plugin install it as you would any other SIMBL plugin.