Mutillidae is a free, open source web application provided to allow security enthusiest to pen-test and hack a web application. Can be installed on Linux, Windows XP, and Windows 7 using XAMMP making it easy for users who do not want to install or administrate their own webserver. Has dozen of vulnerablities and challenges. Contains at least one vulnearbility for each of the OWASP Top Ten 2007 and 2010. Uses XAMPP to switch between secure and insecure mode.
Source: https://thehackernews.com/2012/03/mutillidae-2117-born-to-be-hacked.html