User Review: Sea to Summit Reactor Thermolite Liner
Temperature Rating: adds 10-15 degrees to your bag's rating
Price Paid: $40
This thing is great and unbelievably warm for the size and weight... I carry it in the late fall and early spring to extend my 20 degree bag's range. On nights when I don't need the extra warmth I roll this liner into a fluffed up ball and tie it inside a bandana to make *THE MOST COMFORTABLE* backpacking pillow in the world. In the summer it makes a great lightweight quilt, and it can be used while traveling in hostels too.
pros: tiny, light, and warm...
cons: a bit pricey, and feels somewhat fragile (though it hasn't ripped yet).