UsernameCheck helps you quickly check the availability of your favorite username across 45 popular user-powered websites. In case you want to make sure you have registered your preferred username on every popular site this is the right website to check first. All it needs from you is a username so that it can check for it on all those sites and give you a report like the one below.
From the site:
“This site is a quick and dirty solution to a question that I often lay awake at night worrying about. Do I have my username registered across every site that I should? What if the next internet humiliation meme just happens to share the username I’ve been using for years, and suddenly people are emailing me asking “hey, is this you ???”.

Update: The site is no longer available as an alternative use CheckUserNames.
Go to UsernameCheck @ www.usernamecheck.com (via Webware)