There is an international unicycling club, but for me it's an informal thing, and I'm really not into the organized competition and certification system they have set up.
After living in Ithaca for a while, I've realized that unicycling could also be a way to get in shape, with all these hills all over the place. My goal is to make it up Williams Street (the steepest street in Ithaca). Kudos to Gordon Kindlman (Cornell Math '95), who can actually do it.
Some other unicyclists I know:
Mountain Unicycling (what a bunch of weirdos!)
Several photos from the Fake Foster's Unicycle Run in Boston, MA (April 1999)