Telligent Community is a free commercial community platform that includes a suite of social applications (forums, wikis, blogs, etc.) and social services (likes, ratings, reviews, comments) that you can use to create communities.
You can run Telligent Community side-by-side with Sitecore. Telligent Community is designed to easily integrate with 3rd party Web Content Management platforms. Integration can be as simple as single sign-on (SSO), aggregating Telligent Community RSS feeds, embedding Telligent Community iFrame widgets or can be more natively integrated directly through Telligent Community’s platform API (RESTful web services) and native SDKs designed specifically for Web Content Management Systems (such as Sitecore).
This is a blog post that I wrote for Nonlinear Creations.
April 24, 2018. The source code in the GitHub repository was moved to blog/sitecore/telligentsdk/sitecore-telligent-unencrypted-sso.