Open Source Software (OSS) is created within the guidelines of the Open Source Definition (OSD). It is community based and supported. It's FREE and nonproprietary. The Source code is viewable and modifiable by the public.

Open source is a development method for software that harnesses the power of distributed peer review and transparency of process. The promise of open source is better quality, higher reliability, more flexibility, lower cost, and an end to predatory vendor lock-in. Visit Open Source Initiative

Examples: PHP, MySQL, Drupal, OpenOffice, Apache, Linux, Perl.