Soulver is a new kind of calculator application which uses a simple yet powerful word-processor style interface instead of the traditional “button” approach to doing math.
Update 21/04/10: As itohsnap pointed out on Twitter, there’s one thing wrong in that screenshot. In Soulver’s defence, it gets the right answer if you tell it to do k / q. The fact I didn’t notice is why I need a calculator in the first place.