gistfile1.rb

# Basic tagging system for mongoid documents.
# jpemberthy 2010
#
# class User
#   include Mongoid::Document
#   include Mongoid::Document::Taggable
#  end
# 
#  @user = User.new(:name => "Bobby")
#  @user.tag_list = "awesome, slick, hefty"      
#  @user.tags     # => ["awesome","slick","hefty"]
#  @user.save
#  
#  User.tagged_with("awesome") # => @user
#  User.tagged_with(["slick", "hefty"]) # => @user
#  
#  @user2 = User.new(:name => "Bubba")
#  @user2.tag_list = "slick"
#  @user2.save
#  
#  User.tagged_with("slick") # => [@user, @user2]

module Mongoid
  module Document
    module Taggable
      def self.included(base)
        base.class_eval do |base1|
          base1.field :tags, :type => Array    
          base1.index({ tags: 1 })                                                                                                                                                                  
          base1.attr_accessible :tag_list       
 
          include InstanceMethods
          extend ClassMethods
        end
      end
      
      module InstanceMethods
        def tag_list=(tags)
          self.tags = tags.split(",").collect{ |t| t.strip }.delete_if{ |t| t.blank? }
        end

        def tag_list
          self.tags.join(", ") if tags
        end
      end
 
      module ClassMethods
        # let's return only :tags
        def tags
          all.only(:tags).collect{ |ms| ms.tags }.flatten.uniq.compact
        end
        
        def tagged_like(_perm)
          _tags = tags
          _tags.delete_if { |t| !t.include?(_perm) }
        end
        
        def tagged_with(_tags)
          _tags = [_tags] unless _tags.is_a? Array
          criteria.in(:tags => _tags).to_a
        end
      end
      
    end
  end
end

Updated for rails 3.2 with it's mass assignment protection.
Also the mongoid index method changed a bit:
https://gist.github.com/3313759

Cool, haven't work on this in a while :P.