The Psion 5mx

If you're unfamiliar with the Psion 5mx, you don't know what you're missing:

It's a PDA with a keyboard. (And a damn fine one too, considering the size of the thing.)
The screen has resolution 640 x 240.
It runs for a week of heavy use on two AA batteries.
It uses a stylus on a touch-sensitive screen instead of a mouse. Handwriting recognition is not builtin, but there are 3rd party products.
It has all the usual PDA applications (calendar, contacts, word processing and others), all of which can exchange data with their Windows counterparts.
Lots of freeware, shareware and other software is available.
It includes a sound recorder.
It has 16MB of memory split between a RAM disk and RAM.
You can add a Compact Flash disk in sizes up to 128MB.
And best of all, it's a real computer. You can program it in C++ (on a PC), a proprietary scripting language, or Java. You can load it up with JDK 1.1.4. I have spent many plane rides having fun hacking Java on this cool machine.

You can find out more about the 5mx here.

I'm not connected with Psion or Symbian in any way; just a big fan of the 5mx.