Light, comfortable, multi-purpose - Therm-a-Rest Z Seat

This sit pad isn't complicated or fancy, but it's perfect. It's so light that it's a no-brainer to bring along, even when you're trying to cut weight in your pack. If you backpack where there are generally rocks or logs to sit on, this turns any rock or log of the right size into a comfortable seat. As a bonus, it also makes a nice kneeling pad if you're tending a backpack stove on the ground or if you're building a campfire.

The worst thing that you can say about it is that it's kind of bulky in your pack (although it's still smaller than other backpacking seats that I've seen). But the foam folds nicely onto itself and has a little elastic cord to keep it folded, so it's hard to imagine it getting any smaller. Since it's just foam, you can put it on the outside of the pack without worrying about it getting damaged.