The world’s lightest known solid, Aerogel, weighs only three times as much as air. As air, mind you. You could practically breathe the stuff, but you wouldn’t want to because, basically it’s glass. Really, really, fluffy glass.
It has been used by NASA as insulation for the Mars Pathfinder Rover. Not only is it a great insulator, but it is also made from silicon dioxide, so it can do other glass-like activities. NASA recently used it in the Stardust mission as a collector plate for comet & interstellar dust samples.
The stuff is also incredibly expensive, but that didn’t stop Jason Wells from putting it through the paces. He fried it, shocked it, froze it, baked it, soaked it and eventually smashed it. It’s all highly documented, with pictures, and makes for some good lunchtime reading. Especially since you’re never going to go out and actually buy the stuff.