Do new wheels usually come with freehubs? I was looking around for new wheels and thoroughly confused about the common approach to freehubs.  Do they usually come with the freehub or do I have to actually put that on?  If they do come with it, why in the world do none of the listings actually say anything about sizes?

Specifically, I was looking at this http://www.nashbar.com/bikes/Product2_10053_10052_541505_-1">listing which shows one in the picture, but does not give any specs.
Wheels that take a cassette (Road, Mountain Bike, etc) generally come with a freewheel hub. Generally these are Shimano/SRAM compatible or Campy compatible. If it doesn't say, you can just about bet on Shimano/SRAM compat.

The wheelset you linked to does and is Shimano/SRAM compat.

Wheels for fixies/tracks/single speeds usually do not have a freewheel sprocket (or a fixed cog). If it doesn't say, chances are it's not included.