Technically, nothing since I don't have a tent, but under my bivy I have one of these...

Gossamer Gear Polycryo ground cloth

The page states that it is tough and I have to agree whole-heartedly. They come in packs of two and I've been using the first one for two seasons now. I don't use a groundsheet for waterproofing reasons, but more to keep the stuff that may puncture the bottom of my bivy away.

peace.
ryanc
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Hike Arkansas! and Hike it All!