What is current state of the art for enabling OpenID login in Ruby on Rails applications? This is a community wiki with up-to-date answers to this question.
Authlogic
The most advanced authentication solution seems to be Authlogic. It supports OpenID with Authlogic OpenID plugin. It supports Rails 3.0 and 2.x.
You may want to watch "OpenID with Authlogic" railscast (and the "Authlogic" railscast).
There is a sample application called Authlogic OpenID Selector Example.
restful_authentication
Another authentication library of choice is restful_authentication
Rails plugin. Seems like you also need to install open_id_authentication
plugin.
You may want to watch (old, circa 2007) "OpenID Authentication" railscast.
Ruby OpenID
Raw support for OpenID protocol is handled by Ruby OpenID library.