Elgg is an open source social networking engine. It is easy to build on Elgg because the engine can handles web application and social functionality for you. Elgg need Apache server, PHP and MySQL. The Majority of the web applications are written in the same. Supported technologies are OpenDD, OpenId and OpenSocial.
MegaZine3 , a powerful page flip/ flash book open source engine. MegaZine3 written in AS3 (Actionscript). The basic engine delivers low level functionality like dynamic page loading, plugins provide enhanced features such as an integrated gallery mode. It’s free for private use and open source. The user can flip the page and it uses a completely dynamic approach, allow full configuration through external XML files. See the demo here.
Vanilla is an open-source, standards-compliant, multi-lingual, theme-able, pluggable discussion forum for the web. With over 450 plugins so far, Vanilla is ideal for custom community solutions. Anyone can download and use Vanilla for free. If you do not have a web space, you might be interested in the hosted version of Vanilla as well.