Shazam - listens to whatever music is playing and tells you who it is and what album.
QR Code Reader - free, will read the square bar codes everyone is starting to use.
AccuWeather - free, 10 day forecast and sunrise and sunset times.
Google - free, app will let you search internet using voice. Although if you have the 4s you probably already have this function.
Skype - free international calls over wifi, and free chat.
AppBox Pro - tons of little apps like a flashlight app and a level. Most usefull app is the period calendar, helps to know when to cut my wife some extra slack.
Yelp - free, reviews, searchs things near you.
Pandora - free internet radio.
Netflix - free, can watch streaming on phone.
NFL '11 - free, scores, schedules, game stats, etc.
Dropbox - store files on your comp wireless.
Runkeeper - free, run stats.
Craigslist - free
RedLaser - free, will scan a barcode in a store and price compare online.
Gas Cubby - free, mileage and cost stats. (depressing)
BT App - free, look at your logs.
Bank App for whatever bank you use.
Cover Orange, Angry Birds and Words with Friends for games.
Trip Tracker - free, load flight info and it will keep you up to date on times and such. Useful for traveling and for picking up visitors.
Countdown - free, countdown months, days, hours, minutes to your next race.
Facebook and Twitter free apps, if you use such things.