We have a few different kinds... a lightweight 10x10 Dacron tarp for shade and rain (I can't remember the weight of the fabric, but it's light, and would work as a nice shelter for backpacking) and we also have a 10x10 mid weight poly tarp that I just bought to use as a ground sheet for our new 6 person tent (standard blue tarp - only this one is green).
What are you looking to do with yours?