Kickball is my favorite Foursquare iPhone app
Kickball, by Gorlochs, reduces the number of taps to check in by allowing you to tap and hold on a venue to complete the check in process. Normally you have to select a venue and then tap on the check in button. Yes I realize this sounds silly, but fewer taps makes me happy.
It also supports Twitter and Facebook in addition to being an awesome Foursquare app.