What is open source ? Here is a useful definition and summary. There are many good open source projects on the web and SourceForge is just one good place to start looking.

If your going to incorporate an open source project in your website be aware there are many different types of open source licenses. Some are more open then others. Once you understand the terms of the license make sure your server which hosts your website is compatible with the softwares installation requirements.

Look out for projects which are regularly updated and have a good support forum. Generally, in all newer projects there is little support and your basically on your own in trouble shooting problems. In most cases only the more established projects have an installer which makes installing the software so much easier.