I generally use my Bivy as my primary shelter for most backpacking trips. I bring a tarp to rig up cover using treking polls if it is needed. A Bivy(an OR like DTGuy list above) is always in my truck kit with a good sleeping bag. I also add my Mountain Hardware Bivy to my SAR kit or my day pack when hiking in the winter and in Northern AZ.
_________________________
"Trust in God --and press-check. You cannot ignore danger and call it faith." -Duke