Facebook - I use this all the time
Urbanspoon - I use this once and awhile, great to find new restaurants or to help pick something out for you to eat
Yelp - if you are looking for a restaurant review, this app is golden
Whitenoise - good app to help you fall asleep to. If you like to sleep to crickets or rain this is where to go.
Tapdefense - strategy tower game - position different types of towers on the map to kill the characters walking across
Cannon Game - you are a stationary cannon and your objective is to estimate the right angle and velocity to use to hit your targets
Colloquy - IRC program - great for gmatclub chat
FlightControl - just got this. It has me occupied for awhile. You are a flight control operator and planes cross your screen and you need to swipe a flight direction for each plane to land in correct airstrip. Gets hectic when there are 10 planes on screen, some traveling at different speeds, trying to avoid a collision.
Backgrounds - access to tons of different types of iphone backgrounds.
Fun apps to show your friends -
Snaptell - take a picture of a book cover or CD etc and it will find that item, display the actual picture of whatever it is that you took and give you access to reviews etc
WhattheFont - similiar to snaptell, but you take a picture of a font and it identifies the name of the font. Great idea, havent had much use or luck with this app.
Shazam - this one identifies songs that you may hear during a commercial or at a club