I've phased out all of my Shimano / OEM freewheels w/ a nice collection of White Industries freewheels of different sizes available for gearing swaps between my single speed bikes that take freewheels. Currently, just my commuter and my SSCX bike take freewheels (my SS mtb has a Hope Pro 2 trials single speed cassette hub...mmmm), but I've got at least one WI freewheel in sizes from 16 thru 20. The commuter's got a 17t and the SSCX has an 18t trials freewheel. WI are absolutely the best freewheels available.

I'm tempted to try the new-ish GT freewheel that Dan's Comp stocks these days to see how it is for 30 bucks.