if you have a hosting solution with ftp access, php support, and mysql db, you could use phpTourney; i believe this is used on the cwg cup page. there also exists a free bracket system which is made by putty from challenge-tv, but i can't find a working link atm.
works for me, but if you want a similar solution, i think ggl provides one that's based on putty's script. check out the competitions page: there's an option called "run your own competition" on the right side of the screen. you have to be registered on the site in order to use this service, though, and the same might be required of participants; that might be an obstacle.
just a website so ppl could sign up
yes
e: not :<
don't know of any other alternatives.