Authlogic_RPX is the gem I released some time back that lets you easily support multiple authentication schemes in Rails without really having to think about it (Facebook, twitter, OpenID, Google, Yahoo! etc etc etc).
With thanks for contributions from John and Damir, Authlogic_RPX 1.1.1 is now available and includes an internal identity mapping that lets you tie multiple authentication methods to a single user. There are also some other bug fixes and improvements, so it is recommended you update Authlogic_RPX even if you don't want to enable the identity mapping.
See the Authlogic_RPX repo on github for information about installation, usage, and don't forget the sample site (and sample code).
Under the covers, it uses the RPX service from JanRain, a bit like this:
To get a better idea of why you might want to use Authlogic_RPX, and some pointers on how to do it, here's a reprise of my presentation on the subject to the singapore Ruby Brigade last year:
Soundtrack for this post? (I want) Security Jo Jo Zep & The Falcons. 1977, tight pants, mo and all;-)