ubuntu linux - visible bell or screen flash

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http://superuser.com – I'm using Ubuntu Linux with Gnome. Is it possible to create a visible bell, or a screen flash? I often run programs in terminal that download files or some other process that takes a half hour or so and when they are done, I would like to be notified visually. There is a beep command in the terminal, is there something like this that operates visually in Gnome? (HowTos)