Super light and versatile - Exped Bivybag Uno UL

You can put your bag inside it, you can use it without a bag, and you can use it inside your bag as a vapor barrior liner.
I don't need it this long so I'm considering cutting off the end and putting in a shock cord closure so that you could walk around in it too.
This is the lightest of all the waterproof bivy sacks I looked at without getting into the mylar type bags that are a nightmare to fold bag up. This is versatile for many uses. The Duo version of this bag looks pretty interesting as well.
Also a good thing to stick in your pack in the winter backcountry.