Fair game, although I wouldn't go overboard with it. I would consider the one abandoned by my seatmate as fair game, but canvassing J/F at the end of the flight hoping to score four or five of them as excessive.

I also have no hesitation in taking my unopened hotel amenities with me - we use them for guests at home or give them to charities who collect them - but I wouldn't go into another room to get them. That's just creepy...

I used to keep my plastic cups from sporting events. Still have a stash of mid-1980's Kansas City Royals cups that we use for the kinds of parties where we'd otherwise use solo cups. People like them (perhaps because they invoke memories of *good* Royals teams!) and they reduce waste since we reuse them.