I recently picked up two thinclients to mess around with. I installed freedos on one of them and it works quite nicely. I installed drivers so I can go online with it. These things can run small linux distro's even with gui. These older thinclients can be bought for a very low price second hand. The older ones have many useful interfaces for hardware hackers such as rs232 serial and printer ports!
This is one of the thinclients I got. The other one is missing the case. I will post a pic of it in a bit.
These things are designed to be low power, so don't expect amazing performance from these.
Specs of the Netvoyager Thin Client LX-1020 :
Cpu: 533MHz x86
Ram: 128MB (can be expanded to 512MB using regular laptop ram)
Harddisk: 128MB compact flash card (I think it will take larger cards too, but this came with it. Dos takes just 6MB, so it's fine)
Has an IDE interface, but did not come with a harddisk.
Max resolution : 1600 x 1200 x 32 - bit colour
Interfaces: 4xUSB, 10/100M ethernet, printer/parallel, rs232 serial
Soundcard and built in speakers
Power : 12V 3.5A (label)
Off : 9W
Running : 23W
Dimensions H x W x D (mm) : 173 x 237 x 37
It is expandable with laptop card thingies (no idea what the interface is called)