This extension implements core of the compiz Opacify plugin, i.e. inactive windows obscuring active ones are made semi-transparent. It works best with focus-follows-mouse policy but without auto-raise, i.e.:
gconftool-2 --type string --set /apps/metacity/general/focus_mode mouse gconftool-2 --type boolean --set /apps/metacity/general/auto_raise false
Due to limitations of the Gnome Shell (as of 3.2), the extension is not easily configurable and the inactive opacity is hardcoded at 50%. To change it, edit the line saying:
var opacity_transparent = 128;
to anything you want (0 is invisible, 255 is fully opaque).
You can also control animation speed by setting:
var transition_time = 0.2;
to whatever value you wish (it's in seconds). You may disable animations completely by setting the time to 0.
Note: This extension will probably break anything else that wants to alter individual windows' opacities by resetting them at every window focus change.