I have seen this a couple of times now, but it is a useful service. Now the intended use is to see what sites your user name is available or taken on. I would also point out that it is useful to tell if you have lost or forgotten about a site you registered on.
Robin on TechCrunch has a good post on it – and the site itself can be found here. Check it out and see if your default username is in use somewhere.
Check your usernames