John the Ripper

John the Ripper is a popular open source password cracking tool that combines several different cracking programs and runs in both brute force and dictionary attack modes.

John the Ripper is often used in the enterprise to detect weak passwords that could put network security at risk, as well as other administrative purposes. The software can run a wide variety of password-cracking techniques against the various user accounts on each operating system and can be scripted to run locally or remotely.

Originally developed for Unix-derived systems, John the Ripper is available for most common platforms. The free and open source  (FOSS) version is generally distributed as source code. A commercial version, John the Ripper Pro, is a more user-friendly version distributed as native code for a given system. 

See also: password entropy, password blacklist, password strength meter