So I finally got around to replacing the dummy text in the sidebar with real, working archive links, and it was stupidly easy.  Check it out...

First, I added a helper method to take all published posts and group them by the month they were created.

def archives
  @archives = Post.published.in_order.all.group_by { |t| t.created_at.beginning_of_month}
end

Then I used that helper method to display links for each month in which posts were created.

  <% archives.each do |month, posts| %>
    
<%= link_to(month.strftime('%B')+" (#{posts.size})", archive_path(:month => month.month, :year => month.year)) %>
  <% end %>

You probably noticed the archive_path named route that the links point to. Here's how I set it up.

map.archive '/posts/:year/:month', :controller => 'posts', :action => 'archive'

That route points to the archive action of the posts controller which merely retrieves the posts created within the specified month and renders the list view.

def archive
  @posts = Post.published.in_order.from_month(Time.parse "#{params[:month]}/#{params[:year]}").paginate(:page => params[:page], :per_page => 5)
  render :action => 'index'
end

The archive action uses a from_month named scope on the Post model to restrict the find to all posts created within a given month.

named_scope :from_month, lambda { |month| {:conditions => {:created_at => month..(month + 1.month)}} }

That's it. That's all it takes to set up simple archive links for your blog.