Learn-Ruby-by-Testing-First is a project mainly written in Ruby, it's free.
Inspired by http://testfirst.org and http://rubykoans.com
Topics covered:
operators
arrays
hashes
loops, iterators, enumerable
methods
blocks
ranges
exception handling
member visibility (public, protected, private)
inheritance
unit testing (test::unit, rspec)
closures
basic functional programming
modules/mixins
method lookup
duck typing