I use AIM,Myspace,Mobile Banking, Box Office, and Aurora Feint. Aurora Faint is pretty addicting. And its free. I wasted 10 bucks on monkey ball! Some of the apps are way over priced. So i'll wait a few before i start buying any more.
I have been playing with Shazam and Midomi for a good bit of the day. I found Shazam to be very accurate and have a large catalog while Midomi is quite the opposite. Only cool thing about Midomi is that you can sing or hum songs to it, however it tends to get them wrong if you don't sing perfectly (much like myself) or it will give you covers of the songs you want.
Dynalicious. It's a little performance meter like a G-Tech you can download. Measures 0-60, 60-0, 1/4mi & 1/8th mile times, g-force when accel/turn/stopping, etc. It' costs $12.99 and according to the reviews is reasonably accurate.
The Mrs has it on her iphone and uses it when she's not boring random strangers to death with her iphone photo album. "Here's one of the dog sleeping. He sleeps on his back! Doesn't he look funny?"