Elgg was voted best open source social networking platform in 2008

Elgg is an award-winning open source social networking engine that provides a robust framework on which to build all kinds of social environments, from a campus wide social network for your university, school or college or an internal collaborative platform for your organization through to a brand-building communications tool for your company and its clients.

• Well-documented core API that allows developers to kick start their new project with a simple learning curve
• Composer is the package manager of choice that greatly simplifes installation and maintenance of Elgg core and plugins
• Flexible system of hooks and events that allows plugins to extend and modify most aspects of application's functionality and behavior
• Extendable system of views that allows plugins to collaborate on application's presentation layer and built out complex custom themes
• Cacheable system of static assets that allows themes and plugins to serve images, stylesheets, fonts and scripts bypassing the engine
• User authentication is powered by pluggable auth modules, which allow applications to implement custom authentication protocols
• Security is ensured by built-in anti CSRF validation, strict XSS filters, HMAC signatures, latest cryptographic approaches to password hashing
• Client-side API powered by asynchronous JavaScript modules via RequireJS and a build-in Ajax service for easy communication with the server
• Flexible entity system that allows applications to prototype new types of content and user interactions
• Opinionated data model with a consolidated API layer that allows the developers to easily interface with the database
• Access control system that allows applications to build granular content access policies, as well as create private networks and intranets
• Groups - out of the box support for user groups
• File storage powered by flexible API that allows plugins to store user-generated files and serve/stream them without booting the engine
• Notifications service that allows applications to subscribe users to on-site and email notifications and implement integrations with other their-party services
• RPC web services that can be used for complex integrations with external applications and mobile clients
• Internationalization and localization of Elgg applications is simple and can be integrated with third-party services such as Transifex
• Elgg community that can help with any arising issues and hosts a repository of 1000+ open source plugins