Open Source Software
Open Source Software is software that is free, freely distributable, and provided with its source code. Developers can modify and improve the code however they wish, and freely distribute the modified code.
As a result, open source software has huge benefits. Since it is completely free, anyone can us it. Since the source code is provided, anyone can improve the program - thus creating an even more outstanding program.
Open source software can be developed by for-profit companies, but only if they provide the software for free.
Nonprofit organizations can benefit from many open source programs, because often times these organizations have very little available cash and are looking for the cheapest alternative. However, while many of these programs are cheap (free), they are also high quality and can often times do the same thing, if not more, than conventional programs.
List of some Open Source Projects
Linux
OpenOffice
Firefox
Drupal
CiviCRM
Moodle