Update: Thanks for the links! That being said, I didn't mean parsers written in Javascript or Ruby, I mean PEGs or CFGs that parse Javascript and/or Ruby programs :-)
OMeta is based upon PEGs and is worth a look because it has some great meta-meta ideas from the 60's that were ignored.
(Also, there are several ports of OMeta to other languages)
(Hmm, I must definitely try to port it to PEG.js :-)
As for Ruby, I am not sure if it is even possible to create a PEG grammar. Its lexer and parser are heavily interconnected and there is a lot of state involved. If the grammar will be created, I don't think it would be pretty.
Im writing a programming language in Ruby at the moment - and after playing with that for a few days I've gone back to "hand coded lexer plus Racc for grammar". It's probably personal preference but I find that a little more tunable.
Also peg/leg for C is intended (iirc) as a direct competitor to flex/bison et al.