I poked around StackOverflow and Google, but couldn't find anyone who has put together a comparison of authentication gems or plugins for Rails (I'm looking for something for Rails 3). What authentication solutions are available for Rails 3, which are most popular, and what are the differences between them?
Ruby toolbox has a list of the most popular ones: http://ruby-toolbox.com/categories/rails_authentication.html
You can see that Devise and Authlogic are definitely the most popular.
Personally I use Devise. It works well with Rails 3, is easily customizable, and makes it very easy to integrate Twitter and Facebook based auth.