The fastest may be Silver Maple, but you really don't want to plant one of those... They are serious nuisance trees. Unless you plan to plant it in the middle of a field somewhere away from all structures, you'll regret putting one of those in the ground.

Freeman maples (acer x freemanii) are hybrids of Silver and Red maple that grow faster than red, but aren't as problematic as silver. Lots of people like these. I personally favor red maple... or sugar maple.. but they are relatively slow growing... better trees though IMHO than all of the above.

http://forums.gardenweb.com/forums/load/maple/msg052229575291.html

These are all full-sized trees in time, that will overwhelm your house unless it is a mansion. And even with fast-growing trees you have to wait years for a substantial shadow to be produced. How long do you want to bake, while you wait for the trees to develop? Probably better to pursue an architectural solution, such as blinds, awnings or lath.

Generic Acer Rubrum grows pretty well in my yard.

The Silver Maples are faster though. Not the worst tree. Interesting bark, early flowers, good looking leaves, above average yellow fall color. Just be prepared to remove the thing during your life time.

@Toronado: Have you not had any problems with dropping branches (or sap), or invasive roots? Silvers are pretty trees if viewed from a distance, granted.

But my Dad's neighbor had one when I was a kid and it was always dropping stuff all over our yard and driveway.

We planted a Marmo Maple (hybrid) in our front yard, and are now having to consider ripping it out for the reason that many have mentioned here. It is growing very fast, which we thought was going to be a great idea, but it does have very invasive roots, and is sucking the life out of everything surrounding it. (Completely my fault, the research I did before planting it consisted of "Is it pretty?") We've had it for about 3 years... bought it with about a 4" trunk and around 20 feet high... the trunk is now close to 6" and it's probably closer to 30' now. So definitely a fast grower.