This package implements an algorithm that was developed by Eli Biham and Paul Kocher and that is described in this paper (Postscript, 80k).
The attack is a known plaintext attack, which means you have to know part of the encrypted data in order to break the cipher.
You can take a look at the README-file or the changelog of the package, or just download the sources (Gzip'd tar-file, 170k). Executables for Windows (ZIP-archive, 363k) are also available.
"RElf" has kindly provided executables for OS/2 (ZIP-archive, 231k) and DOS with DPMI (ZIP-archive, 332k). I haven't tested these. Thanks!