Lots of good ideas in this thread so far. I've looked at the silicone-impregnated tarps, but always opted for less expesive alternatives.
Like Longbow, I have a few ponchos that I use. You can find the cheaper ripstop ponchos for about $15 and then spray them down with a silicone spray to make them more weather resistent. Outdoor Products and Camp Inn make several models of coated nylon ponchos that will run between $20-$30. These aren't super lightweight, but they have grommets and are mutli-functional.
In my PFD, I have a reusable, multi-ply isothermal blanket that I installed grommets onto. It'll shelter two people in a pinch.
http://www.hartwellmedical.com/
I purchased an 8' x 25' roll of clear 4 mil plastic sheeting with the idea of making several limited-use tarps out of it. Cheap and durable, I'll use one of these this season. I need to put grommets on this one yet. It measures 8' x 8'.
You can also find coated nylon tarps at REI, Sportman's Warehouse, and other stores that carry a decent selection of camping equipment. These tarps are usually available in several sizes and have grommets on them already. They are decent, but aren't as light. I have one that is maybe about 10'x12', and I have used it a number of times with great success.