Gumstix
I picked these up a few months ago, you can get a lot more info at gumstix.com and gumstix.org. They are an embedded Linux development kit, the parts are basically what you would see in an average PDA (sans LCD) broken out into separate boards which you can plug together in different combinations. The individual pieces are fairly inexpensive but when you get a bunch of them, well, it costs about the same as the PDA.

From left to right:
gumstix connex 400 - 400Mhz Intel XScale PXA255, 64MB SDRAM, 4MB flash. This board is fairly useless on its own as it has no peripheral connectors or wireless, you plug it into the other boards you want to use. There is a version with bluetooth on the board which I did not get. It has a 60 pin connector pictured and a 92 pin connector on the back. Reset button on upper right of board.
audiostix AC-97 - audio board, stereo in/out, power and USB client connection
etherstix - 10-100baseT wired ethernet and power.
cfstix - CompactFlash card slot and power, you can plug a CompactFlash wireless card into it to get wireless.
tweener - This gives you a serial port connection and power. I used it once to configure the ethernet board and then just put the board on the local network.
breakout-gs - This gives you easier soldering access to a lot of the XScale pins, mainly for an LCD. Also has USB client plug and power.
The XScale is a system on a chip, so these boards really have very little on them aside from bringing CPU pins out to usable connectors and any support components needed to do that.
You can only use the boards in particular combinations, for example the wired ethernet and ComplactFlash boards both use the 92 pin connector so you can only use one at a time. gumstix has a board which has both CompactFlash and wired ethernet on it, so if you wanted both you just get another board to do it.
These setups look pretty flimsy just plugged together, the boards have holes in them so you can screw standoffs in and make it physically stable.

This is the CPU board sandwiched between the ethernet and audio boards.

This is the CPU board sandwiched between the ethernet and breakout boards.

This is the CPU board with the tweener and breakout board attached.
I bought this to educate myself on the XScale, embedded Linux and get my soldering skills back. The first thing you learn is that when they mean Linux, they mean Linux, not GNU/Linux. It is a fairly sparse setup, kernel and lightweight command line tools take up most of the 4MB flash. Fortunately they have ssh on flash by default so it was pretty easy to get a board combo on the net and login.
All source is available through a Subversion repository, nice. There is a ton of other software to pick and choose from. Fortunately you can NFS mount more disk space, or get a system with more memory.












