Check out Need for Speed Undercover. I was very surprised that its such a full-featured game. It also takes advantage of the g-sensor on the imagio and steers incredibly well through the accelerometer. The graphics are surprisingly good too.
I generally don't find myself playing games like that on my cell phone, but this one got me hooked!
I found a 'free' version, however due to ppcgeeks rules I will not post link. try a Google search for it.
Another good game is Xtract. This is a freeware game and is available at freewarepocketpc.com
Here's a write-up for it:
http://www.brighthub.com/mobile/wind...ews/34729.aspx
Although, this game doesn't take advantage of the g-sensor, its pretty fun.