I used a blackberry without service, for a while as a PDA. It was practical to have two devices in one for me. You can pick up a older used Blackberry for around $50, depending on which feature you need. Never needed more than a phone book and planner so the PDA would have been redundant.
I did not know
PDA wireless was free?
I have Cingular (now AT&T) and only paid $30\mo, for
the Cingular BIS feature. I have heard Nextel runs about $50\mo but have not looked into it.