Opacity Settings in Metacity

Metacity is the default window manager for the Gnome Desktop. One of the coolest and little known features is that you can set active and inactive border opacity via gconf-editor

Run gconf-editor [alt-F2 and type in gconf-editor]

In the left hand browser expand "apps" then "gwd"

screenshot here

Then edit the keys metacity_theme_active_opacity and metacity_theme_opacity by right clicking and selecting edit then just use the arrows to raise or lower level of opacity.