I went a bit crazy on eBay recently and purchased a whole bunch of second-hand PDAs:
* iPAQ h1910
* iPAQ h4150
* Jornada 680e
* Jornada 720
* iPAQ h3850, h3870 and an h3660 all three of which I plan to give away to prospective kernel hackers.
All of these devices were chosen not only because they are now quite cheap, but also because there are already existing working Linux ports for them. They still need someone around to support them however, and in the case of the 4150 there is some minor bugfixing work to be done in Opie (eg. the record button not working, although this could be a keymapping issue). The 1910 works almost flawlessly, hats off to aquadran for a very complete port and thanks for choosing Opie as well :)
I really like the form factor of the Jornada devices - having a usable size keyboard is excellent; however the screen in comparison to screens on other devices is absolutely awful - blurry, dimly backlit, and the colour response is somewhat patchy. It does not help that the touchscreen on the 720 is a bit worn out also. Still, it's plenty good enough for testing. I was able to run the JLime distribution on the 720 which is available with IceWM or a slightly old version of Opie (1.2.2). On that machine, some features are missing (most notably suspend/resume and sound). I haven't gotten around to trying Linux on the 680e, but from what I can tell the port is a little more complete.
As for the h3800/h3600 devices, well, they badly need bringing up-to-date (to the Linux 2.6 kernel). This needs the help of someone with kernel hacking skills - see my mailing list post for details.