Tranquil PC are an outfit based in Manchester specializing in low-power “green” PCs. While most of their range ship with Windows you can buy almost any of their models sans OS including their “barebones server” which always ships “naked” And here’s their little fanless Atom N270 based PCs (note the OS dropdown box defaults to “None” 
. They used to offer a box with Ubuntu installed by default but dropped that a while back.
