WSL is released under the GNU Public Licence.

Go to http;//wsl.xqoob.com for more details.

NOTE: if after installing WSL you get the following error -
"C:/Ruby/lib/ruby/gems/1.8/gems/windows-pr-0.9.3/lib/windows/thread.rb:59:
uninitialized constant Windows::API::Error (NameError)"
run these commands-
$> Gem uninstall windows-pr
$> Gem install windows-pr