Moonlight is a free and open source implementation of Silverlight, primarily for Linux and other Unix/X11 based operating systems. Moonlight 1.0 was released on January 20 2009 and Moonlight 2 was released December 17, 2009
Moonlight is also usable in others browsers aside of Firefox, as a Gtk+ widget.
Moonlight package has been removed from Archlinux (since ?). Go to the Download page of Moonlight website and download and install the xpi file.