Barb Dybwad provides a very useful list of 26 locations from which content released under the Creative Commons and free licenses (such as GDFL) can be downloaded to enhance blog posts, social media sites, and the like.
Elgg, is an open source social network engine providing out-of-the box functionality for deploying social-aware applications in education, business, non-profit and other environments.
Elgg runs on Apache, PHP and MySQL — the same open source platform that the majority of web applications are written in. Elgg is compatible with enterprise technologies like the Zend Platform and any server environment that can run the Apache web server.
Completely re-writen from scratch, Elgg 1.0 is lightweight, extensible, and easily customizable. The vibrant community of developers is producing new mods and themes that extend and expands the capabilities of the platform.