The Tom Tom app is expensive though isn't it?My top 3 :
TomTom (gps)
iBlacklist
GasBuddy gives a gps-based list or map of the cheapest gas prices nearby.
Guidebook (formerly Conventionist) is invaluable when we are at conventions like Naka-Kon. Especially when events change the morning of, they were great about posting updates.
Dropbox. Too popular? Still can't live without it. The iPhone client actually has decent upload tools, along with viewing. Sign up here and it gets me more storage!