A while back I was interested in learning more about programming compilers, so I checked this project out. I actually added array map, filter, reduce, and sort methods to the language, which was fun and a learning process.
Now it supports comments, file I/O, string-primitives, regular expression support and similar things:
https://github.com/skx/monkey/
Updating the lexer/parser/evaluator was nice and simple once you understand how the various parts hook together. The downside is that if you want to create a standard-library for your language you need to spend a lot of time on it.