I recently bought an Integral Designs SilTarp. 5' x 8' and only 7 oz. Packed, it's about the size of a can of beer. Expensive, though: $60.
I anticipate replacing my regular 3.5 oz. Emergency Mylar bivy with it. Besides the rain protection, it provides nice shade for reading and relaxing after the day's efforts.
I took it for the first time last weekend, and by the third time I set it up I was getting pretty good at it. Used six 2 mm guy cords: four 4' long and two 6'. Nice thing is, it puts my hiking poles to good use for the evening. By adjusting the poles' lengths, you get quite a few options in shape.
<img src="http://www.ridgenet.net/~rockwell/tarp.jpg"width=720>