Think long and hard before you implement something like this on your website. Implementing the bog-standard login system is a one-morning task for any engineer capable of writing a web app. Supporting multiple logins well can occupy teams of competent developers continuously and they will still screw it up. The underlying OpenID/etc protocols are some of the worst designed you will ever have the misfortune of working with. Your users will hate the experience of using them on your site, largely because a) they do not understand them, b) they do not solve a real user problem, and c) when you screw up your implementation -- and you will -- you will lock them out of their accounts.
I might be part of a minority, but I love websites which let me sign up with OpenID/Twitter/Facebook. No need to fill out the same registration forms over and over again or having to wait endlessly for confirmation emails. Moreover no need to remember which username/password I used for which sites, etc.
