Open source web software is when a programs source code is made available to the general public to use and modify for free under certain licenses. Contributions that improve the product may often be added to the core program or made available in the form of add-ons or plug-ins.
Why free? there are 1000′s of communities evolving around the web of inspired and skilled professionals who want to get involved, collaborate, and deliver useful products to others who’ll benefit from their efforts.
Offering quality items for free can be a great way to promote your cause and increase traffic to your site where you may generate other streams of income, advertising, products, services, etc…
Open Source Software is generally issued under the GNU or MIT License:
The web browser Firefox is a great example of an open source product where 1000′s of add-ons have been developed to enhance the program (addons.mozilla.org).