I was reading the Squidu forum recently and I came across a post from Poddys. He mentioned a site called http://usernamecheck.com.
This site is terribly useful. If you are like me, you have joined dozens of different websites and Web 2.0 communities. You probably also typically use the same username.
This website goes through a large list of sites and finds out if your username is taken or available. This information is useful to me because it lets me know where I am a member, and it also has shown me several website that I have never heard of that I can join to build backlinks from.