The stuff I use most are the official Twitter, Facebook and Google Reader apps and DolphinBrowser HD (with the LastPass plugin it's particularly amazing)
Google Goggles is pretty-much essential - Shazam is handy.
From time to time I'll use the Weather Channel App, SpringPad, Tunein Radio.
I replaced the crap calculator with RealCalc
Writing games I use Catlog for log recording etc. etc. too