Chad Woolley's blog
From the Too Useful Not to Blog Department:
open_gem from Adam Sanderson is a new RubyGems Plugin to automatically open a gem's source in your favorite $EDITOR.
gem update --system
sudo gem install open_gem
export EDITOR=mate
gem open rails
NOTE: If you have RubyGems 1.1 or 1.2, 'gem update --system' may not work. See the RubyGems Release Notes for more info.