waze Crowd-sourced traffic information. Data hungry though so be careful if roaming.
Tube Deluxe Sends me push updates on the days I typically travel into town rather than drive - very nice interface and very easy to use
1Password Synced with my PC - I have all my usernames, accounts, passwords in a single place. Protected by a PIN and password as required.
Starbucks The UK version now presents a barcode for payment if you have linked your Starbucks card on their website.
Symantec VIP Access You can use this to replace the physical token devices PayPal and eBay use for 2-factor authentication.
Light by Jason Ting And elegant flashlight app - great with the 4S' LED flash
RedLaser Barcode scanner and price comparison.
Nespresso for on the go emergency coffee purchases

Everything else is pretty much occasional use or games.