Ruby Internals
dev-logger.blogspot.com
dev-logger.blogspot.com
Basically, people learning Ruby need to watch this video, spend a bit of time getting accustomed to how Ruby classes and metaclasses/eigenclasses and such work (really not all that complex), and never be spooked by the idea of metaprogramming (least in Ruby) again.
I've even seen people use it as a kind of framework for writing C code in a ruby style. :)
If trolling through language implementations is your kind of thing, I'd suggest having a look at Lua or Python as well (I never got Perl to configure, let alone compile) - then you can compare and contrast with the way things are done in Ruby.
Lua too, but Lua also has the advantage of being so tiny it's easier to read.