AuthLogic appears to be the new kid on the block and seems to be the next evolution of restful_authentication, easier to use, etc
http://github.com/binarylogic/authlogic/tree/master
Edit: now that Rails 3 is out, Devise seems to be the new, new kid on the block
https://github.com/plataformatec/devise
or I have been rolling my own authentication now with the has_secure_password built in to Rails http://railscasts.com/episodes/250-authentication-from-scratch-revised
Side note: Ruby Toolbox is a great site for finding the current best solution in various categories (based on the number of GitHub watchers):
http://ruby-toolbox.com/categories/rails_authentication.html