ScaleFinder is Mac OS X application for musicians. It allows you to find the scale you're playing on, or the chord you're playing, from nothing but a bunch of notes.
I built this utility for whenever I write a song or chord by ear. It's hard for friends to find the key completely by ear, and chances are, I'm actually playing a well-known scale or chord (No matter how weird you think it is!!). You just tell this simple program the notes you're playing on, and it will search through every key/chord in every key and shoots output in half a second, a task that could take hours.