A quick flight over the MRI codebase (in 30 YouTube minutes) rubyinside.com
Pat Shaughnessy (author of Ruby Under a Microscope) and I recently sat down and took a tour of the MRI Ruby codebase. It's suited to people who've never looked under the hood before, not the C extension and implementation junkies - sorry! ;-) If the MRI codebase intimidates or confuses you, you'll pick up a few things here about how classes and methods are defined and what goes where. So go check it out.