What is Moonlight?
Moonlight was built by Novell in collaboration with Microsoft which provided Novell with test suites, specifications, open source code and Media Codecs to create an entirely open sourced Silverlight-compatible implementation for Unix systems.
Access to licensed Media Codecs (MP3, WMV, VC-1) is provided by Microsoft to Moonlight 1.0 and 2.0 users. The first time that you access a web site that requires these codecs, Moonlight will prompt you to download the codecs from Microsoft and install those on your system.
Developing Moonlight/Silverlight Applications
You can develop Silverlight/Moonlight applications using both the tools available on Windows or in Linux and MacOS you can use the MonoDevelop Integrated Development Environment.