Windows Home Server on an Acer Atom CPU here
My current setup -
Acer AH340 Microserver, WHS V1.1
1x1TB plus 3x2TB disks in the Acer
USB 3.0 PCI-E card in the one free slot
4 bay USB 3.0 enclosure with currently 3x2TB drives installed.
The WHS box also connects my weather station, serves as a test publish host for websites I develop, and runs the following media servers:-
SubSonic (for 250GB or so of audio files)
Serviio (for the 6TB or so of video media)
The above is limited by 2GB RAM limit, fixed CPU, and 2TB drive limit (able to be bypassed with a bit of hacking, but why should I)
HP Gen8 Microserver
WHS V2 or Windows Server 2013
DriveBender or similar to give WHS V1-like drive management
The Gen8 has 16GB RAM limit, replaceable CPU (up to Xeon CPU), dual GB Ethernet, built in USB 3.0, etc. I'll probably keep Serviio and SubSonic also.