RVM is a nice tool that manages multiple installations of Ruby, JRuby, gems, etc. It's very useful if you're working on both Ruby and JRuby projects. http://rvm.beginrescueend.com/

Citrus is a fun parser generator for Ruby. It lets you mix small bits of Ruby code in with your parser grammar. http://mjijackson.com/citrus/

Invoking a method with a ampersand-parameter: http://blog.jayfields.com/2007/01/ruby-invoking-method-with.html

Debugging

http://bashdb.sourceforge.net/ruby-debug.html

You want to run Debugger.start as soon as you can (preferably at the top level) so the debugger can capture the full stack frame.

Rails Development

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails - the Rails "trac" site appears to be obsolete although it has powerful Google-fu
http://rubyglasses.blogspot.com/2009/07/rails-gotchas-undefined-method-expects.html - you need to install the Mocha gem by hand
Also need to install sqlite3, libsqlite3-dev, and the sqlite3-ruby gem.
$ cd activerecord && rake test-sqlite3