3G Watchdog Pro - counts your data in/out on your phone. Very, very good.
Angry Birds - obviously. Ad-free version is less than £1
Car Locator - fixes a GPS lock when you leave your car (works with a bluetooth kit to know when the car's turned off and automatically records your location). Costs a few pound if I recall.
Cloud Print - allows you to print to a printer connected to your PC that has Google Chrome installed, from anywhere in the world. Free.
ES File Explorer - allows browsing of files on phone and also on LAN/FTP sites/Internet. Can't recall price, but very useful
Evernote - the best note-taking app there is. Free.
Friendcaster Pro - alternative to Facebook app and is far better. Costs about £1.50 I think and well worth it.
K-9 Mail - alternative to GMail client. Free.
Google Maps - Free.
n7player - currently in beta, but blows away any other music player. Free at present.
Prey - secures your phone / remotely lock/wipe/activate things on it. Free.
Sickipedia - access to Sickipedia collection of 'dubious jokes'
SoundHound - phenomenally good at listening to snippets of music and telling you who it's by etc. Ad version is free, but pro version is a couple of pound.