John the Ripper
John the Ripper is password cracking software. Initially developed for the UNIX operating system, it currently runs on fifteen different platforms. It is one of the most popular password testing/breaking programs as it combines a number of password crackers into one package, autodetects, and includes a customisable cracker. The encrypted password formats which it can be run against include various DES formats, RSA MD4 and MD5, Kerberos AFS, and Windows LM hash. Additional modules extended its ability to include passwords stored in LDAP, MySQL and others.
John is designed to discover weak passwords from the encrypted information in system files. It operates by taking text strings (usually from a file containing words found in a dictionary), encrypting it in the same format as the password being examined, and comparing the output to the encrypted string.