I've used these though they are a little old now I guess:
Metal case, bare bones just add hdd + ram. Pretty sure they were around $270 a couple of years ago. We use them for display panels to run tv's with ubuntu etc.
Though starting to replace them now with just smart tv's where we can as you can get for around $500 something with an internet browser + ethernet port 32" LCD tv which is fairly amazing when you think about it.
I've also used this bare case:
I like how its metal grill/mesh designed for good ventilation so I've been putting amd itx boards , along with an SSD, skipping the fan as the ventilation is good enough for a very fast startup completely silent solution.
I've been tending to use these amd boards:
depending on the price point I need to build the system at. My favourite thing about the amd integrated cpu's is VT extensions supported so good for cheap miniature test beds (I have one at home to run xen on for test victuals). Along with 64bit support. Cant seem to get both virtual and 64bit support on an atom cpu unfortunately. The GPU would be fine for xbmc 1080p decoding video etc..