Heist: A Scheme Interpreter Written in Ruby
Heist is a Scheme interpreter by James Coglan written in Ruby. Scheme is a programming language that's a dialect of Lisp, was an influence upon Ruby, and still a popular choice for learning functional programming (such as in the classic Structure and Interpretation of Computer Programs).
As well as being used as a stand-alone Scheme interpreter, Heist can also be used as a Ruby library for executing Scheme / Lisp code within Ruby applications.
Other than being of interest to programming language and Lisp nuts, Heist is a great example of not just a complete, well written Ruby app, but a well formed language interpreter too. It includes tests written in Scheme (thus giving the interpreter a real workout!) and the code is generally very well structured. Even if you don't want to download it, it's worth browsing through the Github repository as there's some rather nice programming in there.