For free stuff, we just have a place in the staircase where people drop things that are still good but not needed by their previous owner (mostly books). This works with zero explicit coordination.

I'm kind of amazed/impressed that this works, based on my experience of communal spaces. Don't people ever leave junk that they can't be bothered to get rid of? Does anyone adopt responsibility for getting rid of items that have been there a long time and clearly no one wants?