Freecycling
It has a user interface dating from two decades ago, but Freecycle remains one of the best ways to get to the reuse part of reduce-reuse-recycle. We had an extra bookcase that we wanted to get rid of, and it was reasonably easy to list it on the site. Within a day I had two takers, one who dismissed it after hearing the dimensions. The other picked it up in a van this morning.
There have been various attempts at building better alternatives to Freecycle, but it's hard to recreate the same network that FreeCycle has been building since 2003, with more than 9 million members. That said, this feels like the kind of thing that would work really well as a decentralised app.