Cheap Atom boards seem probably the best solution that doesn't cost a fortune to set up small servers without resorting to very old hardware. My home (Xigma) NAS employs a D410 CPU on a used Supermicro X7SPA-L board and does its job nicely with two RAID 1 sets. The CPU TDP is 10 Watts, plus chipset and disks; being single core doesn't seem to be an hindrance to its functionality.
Hopefully ARM boards will bring even lower power figures, but they're still a bit expensive if one needs many SATA ports. The Helios64 platform looked very promising, but sadly it tanked.
