What was Your Senior Project?

Caydel asks: "I am a third year CS major. This year I have a two-semester senior project course in which I can spend two semesters on a project of my choosing. I want to write something very cool, which at the same time provides quite a challenge to me, and serves a useful purpose; however, I am having trouble coming up with good ideas. For those of you out there who have done a similar course, what did you do? What would you have done differently? Which languages did you use? How many skills, that came from outside of your CS courses, did you use?"