Short Screencasts of Gourmet Ruby, by Avdi Grimm
The Ruby “core libraries” are the standard Ruby classes and modules that are always available without loading any extra code.
Sooner or later you'll want to subclass Array. This episode looks at why that's a bad idea, and presents an alternative.
Hash#fetch is one of my favorite methods. In this episode, I take a look at using it to assert the existence of hash keys.