ANTLR

http://www.antlr.org/

Java parser generator. Includes mini grammar base, as well as several pretty cool applications of ANTLR.

Formerly part of PCCTS. See also SORCERER.

ANTLR has tree grammars which let you use grammar constructs to specify how to walk your AST. It also has a nice notation for building and transforming your AST.

A complete skeleton of an ANTLR based transformation system for ANSI C and C with GCC extesions can be seen at http://www.codetransform.com/gcc.html. Included are a parser, tree parser (to be "grammar subclassed" for your transformation passes), and a tree emitter to take your tree back to text.