I purchased the Montell Breeze Dry-Tec UL Sleeping Bag Cover/Bivy which I used last year in dry conditions on my archery hunt. While it does not have tons of extra head room, I love how small it compacts and weighs only 6.3 ounces!!!
I also have a Tarp Tent Squall, which is a great two-man tent at 2 pounds, and my North Face Corollis for inclimate weather.
Ed