Computers, Gumstix, LEON/GRLIB
I could probably spend my whole life googling for interesting personal computer platforms and still never find all the best ones. Today's discoveries are Gumstix and LEON/GRLIB.
Gumstix sell 600MHz ARM boards with 256MB RAM that have Linux pre-installed, weigh 6 grams, and are smaller than a stick of chewing gum. They have expansion boards to add useful connectors for electricity, USB, ethernet, etc. And they support the familiar free software toolchain for developers. I like these ARM System-on-a-Chip platforms that are so simple they don't need firmware.
LEON/GRLIB is a dual-licensed GPL/commercial VHDL design for a complete FPGA-friendly System-on-a-Chip based around the LEON3 SPARC CPU core. The distribution includes lots of hardware designs -- CPU, interrupt controller, USB, ethernet, and more. They say it's ready to roll on a variety of common FPGA development boards, including the Xilinx Spartan-3 board that I have. How exciting!
I start to have a sense of how many interesting computer platforms there are in the world. I've created a XOOS Platforms Wiki page to incrementally collect more information. Please add any knowledge you have!