Useful rails plugins and gems:

aasm:
http://github.com/rubyist/aasm/tree/master

restful-authentication:
http://github.com/technoweenie/restful-authentication/tree/master

Authlogic
http://authlogic.rubyforge.org/

exception notification:
http://github.com/rails/exception_notification/tree/master

Validatable
http://validatable.rubyforge.org/

haml:
http://haml.hamptoncatlin.com/

active-merchant:
http://www.activemerchant.org/

googlecharts:
http://code.google.com/p/google-charts-on-rails/

willpaginate:
http://github.com/mislav/will_paginate/tree/master

paperclip:
http://github.com/thoughtbot/paperclip/tree/master

liquid:
http://www.liquidmarkup.org/

Highline:

A high-level IO library that provides validation, type conversion, and more for command-line interfaces.

http://highline.rubyforge.org/

Info on creating GemPlugins
GemPlugins

BlueCloth
Ensure the config states:

config.gem 'BlueCloth', :lib => "bluecloth"  
# not including the lib causes double declaration warnings in testing.

Delayed Job (Background Processing for Rails):
collectiveidea-delayed_job

config.gem 'collectiveidea-delayed_job', :lib => 'delayed_job', :source => 'http://gems.github.com'