Moonlight was a free software version implementation of the Microsoft Silverlight web application framework. Silverlight was a web application framework Microsoft tried to push as an alternative to Adobe Flash. They failed miserably. The Moonlight software was abandoned when it became clear that Silverlight was going no-where.
Features and Usability
Moonlight supported version Silverlight v1. All it was good for at the time of its release was making Silverlight websites show a message saying "Please upgrade to Silverlight 2" using the Silverlight framework.
Moonlight was abandoned in 2012. It was no loss, nobody outside of Microsoft cared about Silverlight and no websites of any importance used it.