documentation
http://www.ruby-doc.org/stdlib/ Library documentation for 1.8.x
Programming Ruby, 1st edition covers Ruby 1.6
See the 2nd edition (0974514055) for Ruby 1.8
http://books.rubyveil.com/books/Bookshelf/Introduction/Bookshelf
http://ruby.cenophobie.com/rubycheat.php Ruby cheat sheet
http://www.zenspider.com/Languages/Ruby/QuickRef.html Ruby quick ref
snippets
Ruby Cookbook (0596523696) outline & source code
Ruby translation of the Perl Cookbook
Tk GUI
See also CodeProject/UsefulLinks