Except for the minor detail of being a huge security risk, there are a lot of advantages to having the same username and password for all your online services - easier to remember, friends can find you, etc. However, if you can't remember what login you set up, or if you can't even remember what sites you've patronized, then usernamecheck.com is a perhaps useful little resource.
There are a lot of advantages to using the same username for all your online services. It's easier to remember, and friends can find you more easily. If you're someone who's in this habit, you might want to take a look at Usernamecheck, a site that tells you at which sites your favorite username is registered - it scans ~ 60 "most popular" networks for a given username, returning "available" or "taken". They say that it
The sites on the list are some of the best, most useful places to have an account, so it might remind you of something you've been meaning to sign up for, or it might remind you of sites you signed up for and forgot about.
What I find interesting is that apparently somebody else out there is using my usernames at sites I'd never heard of. hmmm.
Nerdy side note on security: I have a couple core password kernels, which are then permuted at regular intervals, and with varying degrees of complexity depending on the sensitivity of the site. So, all my social-networking sites have a 6-9 character password, while my banking/brokerage passwords are generally 12-14 characters.