Matador pocket-blankets are 90-micron-thick nylon blankets with weighted corners to keep them from blowing away and stitched fold-lines for easy refolding. They're puncture-resistant and water-shedding, and fold up to the size of a couple packs of cards. They come in 44"x55" ($25) and 44"x28" ($20), and have a one-year warranty. No information on the overall weight, but I'm guessing they're gossamer-light. They're hand-wash and quick air-drying.
This stuff is catnip for me, the kind of thing that takes an effort of will to resist buying straightaway and adding to my travel-bag. Certainly, this'd be great for walks in the countryside or the beach and impromptu picnicking while on errands (especially with kids).