Genius.com

Genius (formerly Rap Genius) is an online knowledge base. The site allows users to provide annotations and interpretation of song lyrics, news stories, primary source documents, poetry, and other forms of text. Launching in 2009 with a focus on Rap Music, the site expanded in 2014 to cover other forms of media and a platform for embedding annotations into other websites, such as Pop, Literature and R&B. That same year, an iPhone app was released. To reflect these new goals, the site re-launched as simply Genius in July 2014. https://en.wikipedia.org/wiki/Genius_(website)

Annotation System - compare to hypothes.is