I'd like to recommend the Cocoon Silk Bag Liner/Travel Sheet. It works like a sleeping bag in hot weather. I've used it when staying at hostels as well as when couch surfing so people don't have to hassle with sheets for me. It can be a sleeping bag liner, too. It's light (6.3 ounces), packs down very small (5.5 x 7 inches - it comes in its own satchel) and is really quite comfortable.

-- Hulda Emilsdottir

Cocoon Silk Bag Liner/Travel Sheet
$60
From REI, among others.

Also from Amazon

Cocoons comes in other varieties including a Ripstop Silk version.