Two App Store one's I've tried are iHound & Taptrace. Taptrace Pro is free right now and is the better of the two. Does just about everything that the $99 MobileMe "Find My Phone" does except remote lock/wipe.
If you jailbreak, iLocalis and Cylay are the best. They have all & more features of MobileMe, including rote audio recording of the thief, for a fraction of the cost.