I'm a big fan of "Jerven Fjellduk" (Wolvervine mountain tarp). Essentially a tarp with zippers along the edges, fold it in two and zip up to your favorite configuration (something poncho-like or a bivy bag). Slightly heavier than dedicated emergency bivy bags (such as AMK), but durable enough to be of practical use as tarp, wind breaker, everyday comfort whenever slightly chilly etc.
High quality, the basic model weighs about 650 grams. But it costs plenty of $$$... What can I say, it's a high cost contry (high average income, prices inflated accordingly).
Here's their English brochure for anyone interested:
http://jerven.mediabook.no/5/ They started with the "Original", which was a huge hit about 25 years ago, and expanded with insulated models, bigger models, slimmer models, loose arms you could zip on, extension so you could build a tent out of your bivy bag, blazing colors and whatnot. All high quality.