I went to http://dev.stackexchange.com (I tried this out because I thought maybe there's a SE site for it, but it seems to redirect me to http://www.stackexchange.com) and logged in. I pressed the button that "sends SE information to this site" and I got logged in as... Blondie21?
Edit
It seems to login random names (girlish names) for different people, and it seems to be a SE dev site.
dev.stackoverflow.com
anddev.meta.stackoverflow.com
are already proven to exist.account####
.