Usernamecheck.com – Check your username availability across the mail social networks, blogs and web 2.0 apps from one site.
UserNameCheck – site that tells you which sites your favorite username is registered at. UserNameCheck scans more than 60 networks and tells you if your name is taken. Simply type in your preferred username, and the service will let you know if your username is available or not.
Full list of available sites: 12seconds, Behance, Brightkite, [...]