Games
Solipskier
Angry Birds (right? really.)
We Rule
We Farm
Pocket Frogs
Canabalt
Chaos Rings (if you like JRPGS with less time commitment)
Godville (tough to explain... just try it!)
Flight Control
Apps
Appshopper
Discover - Wikipedia in awesome magazine format
Evernote - Cloud based note application that runs on damn near every device.
iTeleport VNC or iTap RDP, depending on your server needs to VNC/RDP into a windows machine
Social - Probably the best facebook interface out there.
New York Times/USA Today/Various news apps
Twitter, must have, best twitter app EVER.
Flipboard
Misc apps that escape category definement
Epic Citadel
Noby Noby Boy (an app far from mainstream, only if you're willing to delve into the super weird/strange/interesting)
There's more, but my iPad is currently spending hours backing up, so I'll post back when it's done.
A lot of the above are free, and most shouldn't set you back too much. They're all worth it, in my opinon...