The ZPacks Square Flat Tarp is the lightest two-person shelter we know of that is capable of providing bombproof storm protection. It weighs a mere 6.1 oz. or 7.8 oz. with guylines. This is our favorite shelter for going superlight, where every gram counts. Our tests show that the superlight version of cuben fiber used in this tarp is not as durable as the material used in the Hyperlite Mountain Gear Square Flat Tarp. Even so, this tarp performs phenomenally well in all conditions and we highly recommend it if saving weight is your top priority.
Unfortunately, ordering this tarp is not convenient since it is not available from major retailers and only directly from the small manufacturer in Florida (made-to-order, involving long delays in delivery). But, if you can get past the idea of waiting for your tarp, you won't be disappointed.
Check out our complete Ultralight Tent Review to compare all of the models tested.