fogus mentions OMeta, which is amazing in my own humble opinion. You can, extremely concisely, create a parser for a language you dream up. The author of OMeta has an example of parsing JavaScript in 200 lines of OMeta. OMeta can also parse itself in about 40 LOC. This conciseness is the reason Alan Kay's Viewpoints Research Institute is using it for their STEPS project (attempting a full GUI system from "scratch" in 20k LOC). They use it to compile all the languages in the project, and even make use of it in their 200 LOC TCP implementation.

The example gist provided really doesn't do it justice (partly because there's a variant with slightly lighter syntax). Check out the sandbox for OMeta/JS (there are several OMetas, each for its own host language), which has several projects you can check out: http://tinlizzie.org/ometa-js/#Sample_Project