Bump: a cool cross-platform mobile app for file transfer

Having just joined the smart phone wave a few weeks ago with my HTC Evo running Android on the Sprint network, I'm still discovering essential apps. One category is "cross-platform" by which I mean an app that has a version for iPhone also. "Bump" is a simple file-transfer app which has this cool gimmick interface: bump your phone to the other person's to initiate transfer!

FourSquare is another cross-platform app. Basically it is like a location-aware Twitter. You "check in" (aka post) and it senses your physical location. The gimmick is that the server tracks the number of times you check in from a particular location, and awards "badges" based on who checked in the most from that site, or who checked in with the most different sites, etc.