I used to work for a company that built off peak storage heaters (they've been out of business for a long time now.) Most of them use some type of brick to store the heat, ours used rock but it was the same principle. Typically they are metered separately from the rest of the house, so the electric utility can keep track of when the heater is on and charge accordingly.
They can work pretty well, although if you have extremely cold weather they may not have enough heat stored up from the off-peak period to last through the day, and then they have to run the heating elements during the peak period which will cost you more money.
Another thing to consider, if you're thinking about using it in your garage, is you don't want to use spray paint or other aerosols around them - the particles in the air will get drawn across the heat storage bricks and burn into them, making a big stink.
It's been long enough I don't know the players in the heat storage field anymore, the only company I can think of still building them is Steffes.
http://www.steffes.com/
