My most commonly used apps are Out of Milk (grocery/list sorter), Yelp, and the DC Metro app that has eight screens apologizing for the DC Metro before you can actually get to the screen that tells you when a bus is coming.
Also Amazon Kindle, Netflix, barcode scanner, XE for currency conversion shit, Decibel (for the weddings), Droidlight (the flashlight app), Dropbox, Facebook, IMDB (useful for bar arguments), Shazam (which I use all the time to ID songs and actually find it sort of creepy), and Weatherbug.