Pop top pull tabs. I haven't seen a can with those in a long long time, but I still find them. Nails in trees too, but those are harder to pack out. As a camp host I find all kinds of things that campers leave in a vacated campsite, I just store it in case they return for it, after a few weeks I tell the ranger and let him/her worry about it. No one ever leaves anything that interests me, darn it..