Doppelganger.tk is a site that lets you check whether a username is taken across a variety of sites. You can filter for found or not found sites, sort by popularity, filter for a specific site, or only show results for sites you've favorited. It also suggests alternate / past usernames that could be claimed. Username info is cached with a lifetime of 5 hours, so any changes could take up to 5 hours to update. Doppelganger.tk is open source (client server).