It really depends on what your interests are. For instance, some good apps are:
Social networking - Facebook, Twitter, Google+, Google voice, Tango or Skype for vid conference on 3g
Productivity- Evernote, Wunderlist, Genius scan, timecard pro
Utility- Appzilla 2, Converter, Dragon Go!, The Weather Channel
Shopping- Ebay, Amazon
Reference- Kindle app, ibookstore, bible, red cross first aid, appbzr, Pulse
Entertainment- Pandora, Stitcher, TuneIn Radio, hulu+(if suscribed), video time machine
Games- Angry birds, super stickman golf, metalstorm, tetris, bejeweled, fruit ninja, infinity blade, plants vs zombies, harbor master, tiny wingss, sonic 1&2, pocket legends, quarrel
The list goes on and on.
This should definately help get you started. Cheers and Happy hunting!