Most useful Rails plugins, Ruby libraries and Ruby gems? [closed]

Question

Will Paginate - essential for pagination.

HAML and SASS

devise - a more comprehensive authentication gem

Authlogic - Easy Authentication

Rspec - for BDD.

Formtastic - Awsome form helpers.

It lets you do things like this:

<% semantic_form_for @user do |f| %>
    <% f.inputs do %>
        <%= f.input :email %>
        <%= f.input :password %>
        <%= f.input :password_confirmation %>
    <% end %>

    <% f.buttons do %>
        <%= f.commit_button 'Sign Up' %>
    <% end %>
<% end %>

and get decent, semantic HTML: