I'm seeing several references to the Op/Tech raincoat... I'm not sure if it's the same product, but I've been using their Rainsleeve for a while and I'm a fan... it mounts quickly, stores easily, and works well. It's a dogleg-shaped affair, the leg with the drawstring goes over the lens and is long enough for all but the longest teles, while the other part of the dogleg goes over the camera and is long enough to cover your tripod head as well (a ball head anyway... if you've got a three-way with handles sticking out this may not be the case).
A small hole in the plastic stretches over your viewfinder... if you put it on carefully, you can reuse it but if you stretch the hole too much you'll have to toss it after one use or hold it on with tape.
At two for five bucks it has to be one of the best bargains in photo gear... even if you never intentionally shoot in stormy weather, I consider this an essential accessory to keep in your camera bag for emergencies, along with a couple of garbage bags to cover the bag itself and yourself if need be.