MongoTagger

sudo gem install mongo_tagger

class Post
  include MonogoMapper::Document
  include MongoTagger
  key :title, String, :required => true
end

post = Post.create(:title => "Hello There!")
post.tag_list = "monogomapper, is, awesome"
post.save

Post.find_tagged_with("mongomapper")
Post.find_tagged_with("mongomapper, awesome")
Post.find_tagged_with(["mongomapper","is","awesome"])