Tomasz Węgrzanowski (aka Taw) has developed a Lisp interpreter embedded in Ruby called RLisp. It's early days and ultra-alpha-quality, but it's a cool project. You can find the source code here, and Taw is offering free beer for the first 10 developers to write a cool RLisp program. RLisp already has some basic OO support (and even HTTP support) so it's fun to play with.