Need to find out where your chosen user name is registered?
http://www.usernamecheck.com/
EDIT: Whoops, didn't see this earlier; still worth mentioning again
/EDIT
Do you know how many people share you name? (this is only an estimate)
http://howmanyofme.com/
remember kids, when picking a user name, plan ahead, then go to all the websites ever and squat the accounts, so you will always have the name
(but no, really, do plan ahead; and don't put a strain on the poor websites info structure)