I use Tyvek, cut an inch or two smaller than the floor of the tent. I use it for abrasion protection and mud protection only. Tyvek is in general waterproof, but that's not really to protect anything but the footprint itself, since you don't want a footprint that absorbs water (which would be annoying to pack up). Tyvek is cheap, very sturdy, readily available, and relatively light (but quite noisy).
Having said all that, I don't really consider a footprint mandatory. In my opinion, it might be better to buy a slightly heavier tent made with a sturdier floor and skip the footprint. A light tent plus a footprint may weigh about as much as a heavier tent without a footprint, and the former will cost a lot more than the latter.