I don't see why anyone would use most of those unless they needed to authenticate users with a third party in order to use their API. Using OpenID as the primar

I don't see why anyone would use most of those unless they needed to authenticate users with a third party in order to use their API. Using OpenID as the primary way of authenticating users unnecessarily complicates the UI. Offering it as an option, especially the dynamic way Intense Debate does it, can work well. Using one of the other authentication systems when you're not trying to get data from that third party will just confuse the user. Why is this random website trying to sign me into Yahoo?