I've got to get some tarps for my fiance and myself. Since we're going for cheap, we'll be starting out with some "ye olde blue tarps" from WalMart. I think they were $7.99 last I looked at them. Definitely not what we'll be wanting for the long run, but cheap and quick until we can afford better.
They have a 9'x9' and a 10'x8' version (plus different sizes, but we don't want any bigger). Would there be any advantages to a square vs a rectangular tarp? Hung diagonally, their ridgelines are about the same (approx. 0.1 foot difference between the two).