Build your singlespeed bike around Surly's New Quick-Release Free/Free Rear Hub. This gorgeous hunk of aluminum has the capability you need and reliable sealed bearings, too! It sports threads for a freewheel on both sides so you can rock two different gear ratios. Plus, this hub's 135mm width fits most MTB frames and the chromoly axle provides a flex-free ride.
Freewheels sold separately.
This hub is drilled for 32-spoke wheels.