I'm really addicted to bivy-sackin'.
My bivy sack of choice for most of my trekking is the Integral Designs Micro Bivy. I put a custom floor on mine, so it only weighs about 12 oz, and it has the ultra-breathable eVENT top on it, so you don't fester in your own humidity quite so bad.
Perhaps my favorite thing about bivy-sackin' is the ability to just about sleep anywhere.
When we got out of our packrafts for the day near the confluence of the White River and South Fork Flathead River a few weeks ago in the Bob Marshall Wilderness Complex, I found this perfect pine tree: the right slope, a little overhead protection from raindrops, low dead branches to hang stuff on, and an awe-inspiring full perimeter view of the mountains and river.
It reminded me of the old trappers from yesteryear, like Jim Bridger. I bet they unfurled their blankets below pine trees like this one.
Photo: Ryan Jordan, July 2009. Sigma DP2 f/2.8 1/1000 sec.