They've been around for at least a year now, and all my research points to obscuring your brake lights as being very illegal. But I don't care. This is the closest my hatchback will ever get to being the batmobile.
For $7 this 'kit' is basically just a sheet of adhesive vinyl with three different sized versions of the Dark Knight Batman logo on it, which you can trim and use to cover the brake lights on your ride. Every time you stop it will let those behind you know you support vigilanteism in all its forms—that is—if they can still see when your brake lights come on.
You'll need to get two sheets if you want both backlights covered. And for an even more impressive effect, you might want to try applying them to your headlights too. Creating a pair of bat signals that blind oncoming traffic. [Amazon via Bookofjoe]