I love these things. Just roll one up to the wheel, flip a small lever (not visible), then step down on the larger lever a few times to lift it up off the ground. If you have four, put one at each corner and you can push a car all over the garage. Thanks to them, I can easily stick a 3rd car in the back of our 20X25ft garage and park two more cars normally. They are also extremely handy if you just need to lift a low car up a few inches. A set of four was $540 from JC Whitney.

A word of warning. If you order from JC Whitney, you get the 5000 model, which has a 9.5" roller (from memory) and doesn't fit on the 245mm wide tires on the back of an S4/S4s. You really need the 6000 model, which has a 13" roller. I've seen these advertised by Griots Garage. That said, you can use the 5000 model on the wide tires but the tire ends up on the frame of the jack itself.