What's your cell phone's ringtone? What made you pick it?
I've actually been waiting for this QotD, and when it came I was totally unprepared. I created my own ringtone with a waveform editor, some effects, and the Cornelius remix of Moby's "We Are All Made of Stars". I wanted something very understated and minimal.
Anyway, when this QotD popped (lil MMO slang ;), I found that I couldn't upload the .wav to Vox. So I hunted around until I found this nifty little freeware app called Switch - which just converts from one audio format to another rather effortlessly. It's earned itself a place in my Apps folder.