Depends on what you want it to do. I've got quit a few.
Road Trip - Used to calculate MPG and track my fill ups.
Tapatalk - Native iPhone access to this and several other forums I use
iMapMyRIDE - used to track my cycling and running with the GPS
MetOffice - Does what it says
Facebook
VNC Viewer - used to remote access my PC's at home
TouchTerm - SSH access to the webservers running this site
PayPal
Bump
TuneIn Radio - online access to many radio stations
pocket BLU - bluetooth remote for some bluray films on the PS3
HA Traffic - Highways Agency traffic updates
Argos
Amazon
Flashlight
iBooks
Dynolicious - supposed to be able to track 0-60 runs, G's pulled and a load of other stuff....haven't really used it though
Yell.com
Google Earth