We have several of the Seville wire rack systems from Sam's Club.
They hold up to 600lbs per shelf (have not tested that!), and are fully modular so the shelves can be set at any height desired.
Oh, and the best feature is that they are on wheels so are easily moved (yes, even when loaded)
I've got most of mine in the storage building but that rack thing looks pretty intriguing. Some more in the attic above the garage. Not easy getting a set of 18" wheels/tires up a ladder and into the attic. Oh yeah, couple more sets of 33" in the Bronco that doesn't run. Maybe I need to get rid of some of them? Nah. Are three sets of wheels for each car a bit excessive?
I've GOT the metal rack from Tire Rack. Clean and simple, easy to put up. Same problem, 3 sets of wheels with tires and 4 extra tires. Brian is right, 2x4 would be a cheaper solution but definitely not as cool looking.