"Who's in town" app - it's difficult to keep track of where all my friends are at any given time, and many times I miss out on catching up with friends because I don't realize they're in the same city that I'm traveling to.

One could say something like the "location" field in chat programs deals with this, but hardly anyone keeps that current. In the past, I would blast a Facebook status of my new location and hope that people would notice - this sort of worked, but it was by no means perfect (and I have since deactivated my account there)

Essentially, you'd specify a whitelist of people you're willing to share your location schedule with, and the app would alert you if there's overlap.