Monkey Ball is a great game, IMO, and is a good example of where game control (via the iPhone's accelerometers) will go. For me and my daughter, it is worth $9.99. Cube Runner is a very simple and free game that also shows off the accelerometer based control.
There is a free VOIP app, Truphone, but it's still expensive to call and I'd wait on a cheaper alternative and hopefully Skype. Although, in some cases, Truphone may be cheaper than your plan minutes, FWIW.
AOL Radio and Pandora are both very good (and free!), IMO. I think I read that Pandora gives more bits on Edge with first gen iPhones than AOL.
Get the free "Light" program; I actually use this a lot, more than I would have expected.
Facebook is very good as well as Weatherbug and Movies . . . all free.
I'm really looking forward to DataCase - looks like a useful wireless drive app:
http://www.tuaw.com/2008/07/14/turn-your-iphone-into-a-wireless-drive-with-datacase/
I've only been digging around a few days, but these are early goodies, IMO.
Best,