Find My iPhone App
Awesome app (until my lost 3GS' battery went dead). Would've found my phone using it if it weren't lost on the side of a busy road. Not worth a life! Almost though. Allows you to remote lock, send the phone a message if recovered, send a sound (even if muted) and wipe out the phone if necessary. Good stuff.. oh, but you have to be a paying MobileMe customer. Small issue with that but since I am already no biggie.