An emergency blanket or bivy is extremely useful too and weighs very little. I just came back from spending a night in the snow (http://i.imgur.com/yc0cc.jpg sunset, Sequoia National Park, ~8,000 feet). At night, temps went down to the 20s (fahrenheit) I believe and I was shivering pretty bad in my sleeping bag until I took out my emergency blanket. In the morning, I cut it up and wrapped the pieces around my feet before putting on my boots, since I was still using soaking wet socks from the previous day (lost my extra pair somewhere on the trail). The mylar isn't breathable though so one has to watch out for perspiration.