I use a lightweight backpacker's tarp from Outdoor Products. They sell them at REI, Walmart, and just about any sporting goods store. I think I paid around $20 for it. I believe it weighs 8 oz, not ultra lightweight, but not bad either. It's gotten me through some wet weather and I'm pretty...
I'm using the REI Minimalist Bivy. So far I like it, haven't had a chance to use it in the rain yet. It looks like it would do well with moisture. It's very roomy, and weighs very little. I doubt I'll ever carry a tent again.
Here's a bit of an update. I still can't believe this is only a misdemeanor. It's terrorism.
http://www.latimes.com/news/nation/nationnow/la-na-nn-booby-traps-20120424,0,5959281.story
I really can't believe this is someone's idea of "fun"
http://www.ksee24.com/news/local/Men-Arrested-for-Setting-Deadly-Booby-Traps-Popular-Hiking-Trail-148542875.html