The new Intel NUC units (http://www.amazon.com/Intel-NUC5i3RYK-Core-i3-5010U-BOXNUC5i3RYK/dp/B00TG4NO54) are pretty nice, I've actually seen a couple of them used in colocation environments in an effort to be efficient on power.
This particular one, with an SSD should use ~30w...