Facebook connect provides an API for other websites to leverage Facebook logins. Facebook supports OpenID 2.0, allowing an existing account to have an OpenID associated as an alternative login method.
From wikipedia:
OpenID is an open standard that describes how users can be
authenticated in a decentralized manner, obviating the need for services to provide their own ad hoc systems and allowing users to consolidate their
digital identities.
The OpenID protocol does not rely on a central authority to authenticate a user's identity. Moreover, neither services nor the OpenID standard may mandate a specific means by which to authenticate users, allowing for approaches ranging from the common (such as passwords) to the novel (such as smart cards or biometrics).
OpenID authentication is now used and provided by several large websites. Providers include AOL, BBC, Google, IBM, MySpace, Orange, PayPal, VeriSign, LiveJournal, and Yahoo!.