Elgg is an open-source social networking engine that provides the foundational framework for building custom social networks and community applications. It offers developers a robust set of tools and features, including user management, activity streams, and customizable profiles. Launched in 2004, Elgg is known for its flexibility and extensibility through plugins, allowing developers to easily tailor the platform to specific project needs. The platform supports various use cases, from simple community sites to complex enterprise-level applications.