Moonlight is a open source implementation of Microsoft's Silverlight runtime. It is currently in testing and shall hopefully be released later in the year. Moonlight currently still needs the mono and olive packages built from svn to create a working plugin.
Moonlight needs to be built from AUR, this will either mean downloading each of the packages separately or downloading and installing a program such as yaourt which can get them for you.
The four packages needed are mono-svn, monodocer-svn, olive-svn and moon-svn.
The install procedure for all four packages is the same, untar the tarball, then as root run
from within the newly created folder.
Follow the above procedure for downloading yaourt and then download moon with
#yaourt mono-svn monodocer-svn olive-svn moon-svn