I've tried to use a waterproof parka, but I found myself sweating so much that I was just as wet on the inside as if I wasn't wearing it. I'm a poncho guy.

I use an Exped Bivy Poncho - mostly for the length (I'm quite tall), but a side benefit is that it has all sorts of snap-up and tarp options.

I find using the hood to be too warm too. I'd rather wear my Tilley synthetic fabric hat - it keeps rain off my glasses too.

I recommend Campmor's Cascade II nylon ponchos to Scouts and leaders in my son's troop. They are reasonably priced (though not 99 cents), well made, and come in several sizes, including an extra long-backed version for covering backpacks.

They also have nylon loops in the corners to allow them to be used as tarps. Slick.