Is there a way to set network proxy system-wide?

If I want to have GNOME applications (as well as Firefox and Chrome) access the network through a proxy, I need only use **gnome-network-properties** (a nice and simple GUI I must say). For other apps (e.g. APT, Transmission, XChat), I have to use their specific ways of doing it. Is there a way to avoid this, something I can turn on and off when in a network that requires a proxy (hostname:port)?