There are a lot of virtual accounts we made when importing the mailing lists and forums. If you want to keep your post history, and/or your email address is in use when you go to sign up, you should take ownership of that account.
If you had posted on the mailing list or old forums we might have made an account for you when importing all the old posts. Try to log in with your email address and click "I forgot my password" and you can take ownership of the account. We used the latest email address we saw for you.
If you only posted on the forums we probably made an account for you with a bogus email address. If you search for a post you made, you can tell @icculus (or email@example.com) and he'll fix it up for you.
If you don't care about keeping ownership of your old posts and you don't use that email address anymore, you can always just make a new account, but we can fix up most problems.
Once you have control of an account, you can change its username to whatever you like. We picked a reasonable default from the imported archives.
Problems? Ask @icculus (or firstname.lastname@example.org if you have no account yet).