Extra Food - Not the food we plan to eat. It is an emergency ration that can sustain us through a cold night. Oh and by the way, leave the low carb food at home.
Map - Many people who have been rescued could have easily gotten themselves out of trouble by simply having a good map. With the best of intentions and a superficial knowledge of the area, almost anyone can become disoriented and lost under some circumstances. Just a wrong turn or a sudden snow squall can put you into a completely unfamiliar world.
Compass - Carry a compass at all times. Use the compass and a topographic map and you are on your way to finding that never-done route to that never-visited part of the mountain. You can do all this in zero visibility of course, since the compass doesn't lie when the visibility goes to zilch. For all of you GPS carriers, don't even think a GPS is a replacement. Don't trust anything with a battery in the backcountry.