I get asked a lot about tarps and while I think it's always better to order from one of the terrific vendors on HF, I would be remiss without mentioning this.

http://www.kelty.com/p-140-trail-dome-6-footprint.aspx

This is a silnylon tent footprint that's roughly 9 feet 6 inches x 9 feet 6 inches. I got one and used it as a tarp for my daughter's hammock, turned it diagonally and it worked great. It only has pull outs at the four corners, but for $20, you can rig something (like a contact lens case screwed into the fabric with a pull cord in the top). It's not tremendously light, but at 23 oz., it's not exactly a bowling ball either.

Makes a good option for a NewB who doesn't want to spend too much or for a backup that's not a cheap blue poly tarp.