I use those plastic Kway bivyy bags you can pick up for R30. But here's the trick - I cut one of the long sides open, so it becomes a kind of cape. This idea I owe to the Nepalese. Trekking in the monsoon months there I grew very disillusioned with the flimsiness, lack of waterproofing and overheating of rain jackets or ponchos. The locals use big plastic bags which they then cut open length-ways on one side. Since you are somewhat bent over when carrying a pack, it hooks on your head and drapes over behind. This way you stay mostly dry and don't overheat. I started using this and found it ingenious, simple and robust. In hard rain you simply hold the front closed too. Used it during snowfall in Nepal too. Most of the other trekkers walked by with snow heaping up on the hoods on their heads, on their shoulders and on their packs. With my plastic sheet I didn't have any of those problems and stayed perfectly warm and dry. You can tie the long sides like a belt in front of you, or tuck them between your back and pack (admittedly the Kway bags are a little too big for this tucking to work effectively, but the sheets I had in Nepal worked perfectly).
So, when a thunder shower suddenly hits in the Berg, I whip out my red cape, fling it over me in no time, find a good shelf or step in the grass I can sit on, and wait it out. It wraps right over my pack and underneath me and I stay perfectly dry. Can even accommodate more than one person. Have endured many a storm this way. If it hails you can prop up the cape with hiking poles (so you don't get painfully pelted). And if you need to open your pack in pouring rain, you just drape it over everything. I also most certainly also use it as a tent footprint.
As a groundsheet in a cave it works either folded double, or folded open. If folded open it's ingenious in that it forms a kind of a boat on one of the ends and will keep your sleeping bag well away from dust and damp. If you need extra insulation or to keep drips off, you can still fold it over you (more or less like its original design).
For me its an essential part of my gear and well worth the 400 odd grams it weighs. If it works for you too, great! Only in the drier winter months do I swop it out for one of those space blankets with the plastic backing (this kind of a sheet will also work, by the way, but you don't get the hood or cape effect).
