http://go-mono.com/moonlight/
http://www.mono-project.com/Moonlight

Moonlight is an open source implementation of Silverlight, primarily 
for Linux and other Unix/X11 based operating systems. In September 
of 2007, Microsoft and Novell announced a technical collaboration 
that includes access to Microsoft's test suites for Silverlight and 
the distribution of a Media Pack for Linux users that will contain 
licensed media codecs for video and audio.

Moonlight 2 was released December 17, 2009