Found this morning via a Very Short List email:
Is your user name — the one you always register with online — unique? Use the Username Check to find out. The application searches Yahoo, eBay, Flickr, Tumblr, and 56 other popular websites, and tells you whether anyone else is using your favorite alias. If your name is available, sign up! — even if it’s a site you don’t use. (You don’t want to see some twit Twittering in your name, do you?)
Very handy for keeping track of where you have (or have not) registered.