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 is currently under development.
Moonlight is also usable in others browsers aside of Firefox, as a Gtk+ widget.
Moonlight is available in community repository, for installing:
pacman -S moonlight
If you want to try Moonlight after the installation, visit this web: Moonlight 2.0 test sites