I'm running Mint 17 with cinnamon, I know you can "Always Top" a window, but I can't find a way to "Always Bottom" a window.
In fact, I'm running a fullscreen app (remote-viewer), but I wish all other applications never go behind.
I'm finally using devilspie and gdevilspie with a config like this:
( if
( begin
( contains ( window_name ) "something_in_windows_name" )
)
( begin
( fullscreen )
( below )
( println "match" )
)
)
it will set the windows with "something_in_windows_name" to fullscreen + force below any other (except the background with icons etc)