In reply to ajsaddacs: It really depends what scenario you envisage you might encounter, which you don't say.
That can potentially vary, depending on what activity you are doing (walking, ski touring or climbing), whether you are going solo, in a pair or in a group and in what capacity you are doing it (recreationally or professionally).
However, in pretty much every scenario I would pack a group shelter as my primary emergency equipment (unless carrying tents and sometime even then).
In addition to that I, if I was not carrying a proper (goretex etc.) bivvy bag with or without a sleeping bag, I would generally carry one of the two items that have already been recommended:
Adventure Medical Kits emergency bivvy
Blizzard Bag foil sleeping bag
So:
Going climbing as a pair, I'd take a 2-man group shelter and have an AMK emergency bivvy in my first aid kit.
If going out in a group, I'd take a larger group shelter (if not two) plus either a Blizzard Bag or proper bivvy bag and sleeping bag.