I carry a 11x17 padded shipping envelope (the plastic ones, with the "bubble wrap" inside). They have many uses:
- A cushion, great for sitting next to the campfire. It also insulates your rear end if the ground is cold.
- A cooler, great for keeping that end-of-the-day 6 pack cold from the depanneur to the campsite.
- A thermal insulator, great for preventing certain things on hot days(insulin comes to mind) from becoming too hot.
- A padded protector for sensitive equipment like SLR camera lenses, etc.
- You can also fold it and slip it under your Thermarest when your tent is on a less than perfect surface.

The envelope I use is a Jiffy Tuffguard cushioned mailer #6 from Sealed Air Corp.

I'm sure you can come up with other creative uses...