Moonlight is open source implementation of Microsoft Silverlight and now its Preview is available primary for Linux and other Unix/X11 based operating systems. There was a official announcement available on Silverlight blog.
Silverlight is Microsoft's web plug-in to author rich web application and it is competing directly with Adobe Flash. Availability of Moonlight on Linux can be seen as a push form Microsoft for Silverlight technology to extends its support to different platforms other than Windows. To get side by side view of Flash and Silverlight features, find comparison chart.