PEG (Parsing Expression Grammars) and packrat parsers are described in The Packrat Parsing and Parsing Expression Grammars Page.
A darcs repository is available at http://subvert-the-dominant-paradigm.net/repos/cl-peg/
- Tony Garnock-Jones has a Scheme implementation of a packrat parser (paper)
- Alan Post has a Scheme PEG parser generator called genturfa'i
- Jon Rafkind has Scheme packrat parser generator called Pegs.