If you're a social networking gadfly like me, you sign up for every new site that comes online just to reserve your preferred username. My motto is "Register now and check features later." It's tough to build a 'personal brand' if you're known by different usernames on different sites.
But what about the sites that somehow slipped past your elaborate internet early-warning system? If your preferred username is gone, there's little you can do about it (unless you're a celebrity and someone is impersonating you), but there's a new tool on the web that can help you find sites where your name is still available: Namechk.com.
Namechk.com is one of those sites that does one thing, but does it well. You enter the username you're interested in, hit the "Chk" button, and the site will poll 88 (currently, and yesterday there were 84 so the crew is working hard adding new sites all the time) social networking sites to see if that username is available. If it is, the site is just a click away; better go register!
As an added bonus, most of the sitenames have a tooltip describing what they're all about, making Namechk.com a fun starting point for finding new social networking sites.