Most people tend to register the same username when signing up for services, for obvious reasons. Your username is your personal identity and most people don't need more than one. It's also easier to remember. But one thing that is becoming increasingly difficult to remember, with so many new Web services sprouting up ever day, is which services you have signed up for exactly.
A recently launched application called Usernamecheck gives you a great overview at which Web 2.0 services your ‘default' username has already been registered.
In just a couple of minutes, you'll know just how hung up you have been on trying out every new service on the block, and which one you've once signed up for but have long forgotten about. Or, alternately, where your username has already been taken by somebody else.
Update: the service returned correct results when I tested it, but several commenters are pointing out inaccuracies.
Usernamecheck currently pings 68 services for the username you want to look up, and lets you know instantly if it is still available or not. According to the counter at the bottom of the homepage, over 110,000 user names have already been checked with the service.
Now if you'll excuse me, I have some holes to fill.