Do you have some special source of mechanical energy for this? Maybe you plan to combine it with exercise? Any exercise bike, stair-stepper, etc. already does a perfect job of converting your work into heat.
If you have some other source of mechanical energy, you're still in luck since any materials are basically 100% efficient at taking the work done rubbing them together and converting it to heat. So all you need are things that
1. Don't tend to seize up (would require too much force)
2. Don't slip too easily (would require too much speed)
3. Don't wear out too fast.
Of course the brakes on cars meet these requirements, as do many other things. If you use car brakes, make sure that they're free of asbestos.
If you want to get good energy efficiency (other than via combining with exercise) what you want is a heat pump. These are driven by electricity, but they get much more than 1 J of heat to where you want it per J of electrical energy used. The efficiency of a good commercial unit might be about 5 times better than that. So if your mechanical energy came from a windmill, you'd be better off using it to drive a heat pump rather than just to directly heat, even if there were a couple stages of electromechanical conversion. The same goes in principle for exercise machines, but I don't think it's easy to get parts to hook them up to feed power back to the electrical grid.
As for having the surfaces react to generate more heat, that just basically means that you're also heating with chemical fuel. That's a rather traditional method. The surfaces will get used up, just as a stack of wood would.
(published on 04/08/11)