AntConc

From the website: "AntConc is an easy-to-use freeware concordance program for Windows (98/Me/2000/NT/XP), Macintosh OS X, and Linux. It was originally developed for use by students in the classroom, but also serves as a comprehensive text analysis tool kit for researchers. AntConc is written completely in the Perl 5.8 programming language using ActiveState's excellent Komodo development environment and continues to be developed through feedback from users around the world."