Slightly used Palm M500 that I purchased off of Ebay ("buy it now") for about $17 if I remember right.
I rarely use it nowadays, actually. Other than the stock software, I have Copilot, Ultranav (for the Lear), a universal unit converter (i.e. distance, time, temperature, etc.), and a bartender drink-mixing recipe program. When I flew GA planes I used Copilot for flight-planning and fuel planning all the time. Now, I mostly use the calculator feature.
If I had a Palm with wireless capability, I'd probably use it all the time but I haven't really looked seriously into getting one yet.