Icopter is a free game and I think it's pretty good, obviously there's the MSN and Facebook apps which cut time down on accessing them. Hmm there's an app called Unblock me which is pretty challenging, and it's another free one.
Wow the unblock me game got me addicted to it for quite a long time. Still love it.
Monster trucks used to be a paid app, but now it isn't. I downloaded it yesterday and I'm enjoying it. If you're into photo editing like me, I say you should go check out Adobe photoshop express....it ain't bad for photo editing on the go.