The sitpad use of the envelope works rather well - came up with it a few years ago teaching winter survival on a shoestring.

I found such an incredible array of uses for these envelopes that I now keep a few (pre-crushed for suppleness) in use in my packs as stuff sacks - twist and gooseneck closed and tie.

Here's another example: Cut off the self-sealing flap and cut open the bottom of the envelope. Slip over boot/shoe, gusset, and secure the gusset just "over the hump" of your calf with the 1/2 self adhesive flap and at the ankle with the other 1/2. Instant gaiter. Many variations just on that use - rest is left to the user's imagination.

I found that the key to unlocking my imagination on uses is the transformation that happens when you take that smooth, stiff, shiny envelope and crush the snot out of it until it's supple - uses start leaping to mind. And testing this out is "free" if you swing by your local USPS office...

I look forward to hearing what uses others come up with that haven't occured to me yet.

Tom