Now this is what true computer hacking is all about. Make your computer beep at you or flash keyboard lights when your firewall drops packets. It's a simple one, but I think it has a high wow factor, especially for non-admins and non-geeks. Maybe I'm just easily entertained.
The Spinning Cube of Potential Doom is another nifty visualization for firewalls.
I like the beeping one. I'll have to try that out on my server.