With weather gradually warming after the winter solstice, it might be time to think about getting out of your tent and under a lightweight tarp. The cheapest alternative is the (usually blue colored) plastic sheet with grommets, available from hardware stores everywhere. These are actually not bad. They are waterproof and lightweight when dry. Three drawbacks are lack of durability, long drying cycles, and radically increased weight when wet.
Silnylon (silicone-impregnated polyamide) is a more expensive alternative. These tarps are lighter and quieter in the wind than urethane-covered nylon, but they take longer to dry in the sun, and suffer UV damage almost as easily. All types of nylon stretch after pitching, especially in the rain, so you need to tighten them up in the middle of the night. Suppliers say silnylon is self healing, although a general quality decline has led to "misting," in other words, they might not really be waterproof. Old-fashioned urethane-coated nylon is costly nowadays, but silnylon tarps are probably not worth the price premium if you can find coated nylon tarps in a surplus store. Better would be urethane-coated polyester, which weighs more than nylon, a reasonable trade-off for less stretchiness and greater UV resistance.
One recent development is Cuben fiber, made from Spectra or Dyneema (same thing), a polyethylene fiber with molecules directionalized for extra strength. This is half the weight of silnylon, currently at four times the cost. Another recent development is Spinnaker Ultralight, a thin polyester fiber woven ripstop-style then silicone impregnated. With more reasonable cost, this is probably the best choice now for ultralight tarps.
For interesting background reading about high-tech fabrics, see this page at Mountain Laurel Designs.