I run a N40L with esxi on it (free) and then run multiple windows/linux/bsd VMs on that - one is even my router. One is my NAS, then test and play VMs. There is even a hack out there to be able to run OSX stuff on esxi
Its very quiet! Small little box 10.5" x 8.3" x 10.2"
It only uses about 55w (have it plugged into my killawatt meter), and I have 4 drives in it. Bumped the ram to 8GB, which from reading can go to 16GB even though says 8GB max. And added a 2nd nic (so could use as my router), Sure you could run whatever linux/bsd/windows OS you wanted on it baremetal. But I love the VM aspect of it - My nas VM has raw access to the disks, and I get 70-90MBps transfer rates to and from it.
I would highly recommend it - and cheap. Currently on newegg for $319 (2gb 250GB hdd)
Its very very quiet!! It sits about 3 feet from my chair in my computer room, and never even notice it on other than the lights on the front