I have never used one before but check out the Wild Things Bivy Shelter:
This exciting new product for 2007 solves the age-old question….should I bring it? Designed by AMGA guide Jim Ongena, this bivy shelter weighs in at 13 ounces and will shelter four people in a storm. It can also serve as an overnight shelter for two in a pinch. This should become standard equipment for all outdoor leaders and guides because it’s so small, light, and useful you’ll never leave home without it.
Standard:
Silicone Impregnated Ripstop Nylon
Weight: 13 oz (0.37 kg)
$150.00
Ultralight:
Cuben Fiber Reinforced Spinnaker Fabric
Weight: 3.8 oz (0.11 kg)
$225.00