I quite enjoy hitting the rear fog switch when getting tailgated. You'll often see someone back off a bit when they think you've hit the brakes.
What I really need is to be able to activate the brake lights without braking though. That would be even better. Also to make the car nose dive at the same time so it looks like I'm slowing down but I'm not.
You could always go for a tactic a mate of mine used. A woman in a freelander cut me up quite badly, I missed her by about 6 inches at the most. She then changed lanes again to go in another direction and as I pulled up along side her my mate gives her the 1 fingered salute. She looks across at him, face goes totally white, then BANG! She crams into the stationary car in front of her at about 10-15mph. On reflection she was driving so dangerously that she'd have hit the car in front anyway, but probably at a lower speed.