ECMAScript 5 PEG parser: http://boshi.inimino.org/3box/PEG/build/ES5.html
(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.