Rails 3 authentication solutions
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.
For a Rails3 App definitely Devise ;).Devise is the only authentication system which provides security on all the 3 stack layers of rails: - In 'M', 'v', and 'C' and hence the best to go with. But you got to learn more on how to customize devise to custom fit your application's need. You can find help on customizing in this page https://github.com/plataformatec/devise/wiki/_pages
In Rails authentication from scratch is dead simple to do.Ryan Bates covers this here http://railscasts.com/episodes/270-authentication-in-rails-3-1