You do need to learn to take advantage of materials when the situation presents itself. Take those leaves your neighbor so kindly shredded and deposited on the side of the road. If you're shy wait until the dark of night to go leaf hunting. If you're having a tree removed, ask for the extra woodchips in the truck. If you have your own shredder, pick up your neighbors branches left on the side of the road.
Plants are a different story.
Make sure you know what you're accepting. I've turned down numerous offers of mint. It spreads like crazy. Some plants aren't easily transplanted, so make sure you have a place for them. On the other hand, if you've been wanting a particular plant, don't be shy about accepting a plant from a fellow gardener.