1) An emergency bivy sac: Basic version: 2 extra-large orange garbage bags and emergency blanket, Better version: Adventure Medical Kits Emergency Bivy bag 3.8oz, Best for winter: Sierra Designs Baffin Bivy bag 17oz. In winter I carry the Sierra Designs version, but in the summer months I can get away with the lighter options.
2) Insulating Pad-Bring some sort of pad layer to insulate you from the cold ground. I bring a rectangular piece of blue foam that fits perfectly in the back of my pack or a full length piece if I am going out for a long tour. I use it to sit on for lunches and have it for emergencies as well.
3) Extra Base Layer, thick toque and mitts, and an extra pair of socks.
4) Down booties-Any brand will do. They are worth their weight in gold.
5) Down Pants-The Mont Bell Ultralight Down Pants 5.1oz are very lightweight. I carry the Western Mountaineering Flash Down Pants 6.5oz
7) First Aid kit-Basic items-Triangular bandage, Tensor wrap, Athletic Tape, Pain Meds, etc
8) Survival kit items: Swiss Army Knife with saw, lighter and matches, Tinder-quik fire tabs (tinder), Hot Paws hand/toe warmers, extra food, emergency blanket, water purification tablets, whistle, headlamp, extra batteries for the headlamp and GPS, map and compass (know how to use it!), GPS (ie.Garmin Map 60CSX), cellphone and/or personal locator beacon (PLB)/SPOT.
**N.B. if heading into avalanche country, you will also need to carry a beacon, probe and shovel. Please make sure you take an avalanche course to ensure you know how to use this gear!**
A fantastic resource to get you thinking about backcountry survival is the new book entitled "Survive!" by Les Stroud (host of the show Survivorman).