Academic Integrity: A Guide for Students (from the Office of the Dean of Students)

...that have been successfully used at Purdue...

•  ANTLR

http://www.antlr.org/
"ANother Tool for Language Recognition, (formerly PCCTS) is a language tool that provides a framework for constructing recognizers, compilers, and translators from grammatical descriptions containing C++ or Java actions [You can use PCCTS 1.xx to generate C-based parsers]."
•  JavaCC

http://www.metamata.com/JavaCC
A Java infrastructure for generating parsers.
•  Lex, Yacc, Flex, & Bison

http://www.combo.org/lex_yacc_page/
Old time favorites to "Read the source program and discover its structure."
•  PCCTS

http://www.polhode.com/pccts.html
Purdue Compiler-Compiler Tool Set