1. Ruby http://tryruby.org
2. Hpricot http://hpricot.com
3. Regular expressions http://rubular.com
4. Exceptions http://hoptoadapp.com/
5. Application Performance http://newrelic.com
6. Ruby Doc http://www.ruby-doc.org/
7. Rails API http://api.rubyonrails.org/
8. Ruby gems sources http://rubyforge.org / http://gems.github.com / http://gemcutter.org
9. Listing remote gems from source gem list --remote --source http://gems.github.com
10. List of Rails Plugins http://wiki.rubyonrails.org/rails/pages/Plugins
http://www.agilewebdevelopment.com/plugins