I use the
iPAQ H6315 from T-Mobile but they aren't really advertising it because of all the issues it has.(seems like there was a thread on fatwallet about it going for almost free) I haven't had any major problems with it, so I'm happy. If you are serious about getting one of these expect to pay about $500. T-Mobile should be coming out with a new one any time soon.
oh, iPAQs use windows based OS. Not sure what blackberry uses (probably blackberry). Palm uses palm. I know windows so I went with a windows based system.