Comment on Stoplight

Woefully inadequate description...
Basically, it lets you change how the application behaves with standard close-window (quit-on-last, hide-on-last, and normal) and zoom (normal and full-screen) behavior.
For example. You can have a program automatically quit (not just "close") when the last window is closed. A good example of this is TextEdit.app or Quicktime: close the last window, and it just sits there. With this, you can have it quit completely. Or hide it when you're finished, so it takes less resources to keep it running (handy for those who have transparent-when-hidden dock settings).
You can also modify how the zoom button behaves, between an OSX default and "fullscreen", which forces it to zoom to the screen's boundaries instead of the app's specifications. Handy for those who like to browse in a full-size browser window, or for making a large Finder window in a snap.

I, personally, really love this app. SIMBL rules.