I've got a 3/8" snap-on cordless impact gun (12V). I've had it about 4 years. It gets used every day at work and hasn't given me any problems yet. It can handle most bolts, but bigger stuff like lug nuts and cross member bolts would need its big brother
I also have that little screw gun that Nasty posted a pic of...love that thing.
If you're looking to do cheaper than snap-on, I'd look at the IR ones like Fast92RS mentioned. A guy I work with has one of their 3/8" cordless impacts and its pretty stout. I don't have any experience with the Craftsman pro series, Brian seems happy with his though.