I get a lot of funny looks pulling two or three trash bags out of my pack. My hiking group has a lot of "yuppie hiker" types - great people, with (evidently) plenty of money to buy the latest stuff sack or matching pack cover. In light of the last thread I looked at where trash bags were used to keep the pack dry, here's the one to talk about all the other things you can use that Hefty bag for.

I have used one or more of these for:
stuff sacks
pack liners
drop cloth under the hammock for standing around in sock feet shaking bugs out of my boots
temporary poncho for a dash to the cathole when I've loaned my poncho to someone (or left it somewhere, more likely)
stuff clothes in to make an extra layer of cushion under the sleeping pad
stuff pack in
water carrier
vapor barrier
tote Nalgenes back from the lake after a filtering expedition
collect firewood
collect leaves (though they didn't quite make it to being used for insulation, the pad turned out to be warm enough).

Some uses are more useful than others. White trash compactor bags make pitiful ghost costumes.