Designers act like scientists in some ways. We like to constantly experiment with the use of materials in unexpected ways. And we like to think we have done mankind a great favor when we turn the mundane into high art in those successful cases. Ha, what wishful thinking...
But seriously, even if these following examples will not change the world in a huge way, they'll at least have you walk away going "Why didn't I think of that before?".
Well, that's how design works, right?
Magazine racks and seating can be made out of old carpet samples.
Books For Books is a a bookshelf made out of books. Yes, you'll need plenty of old ones.
Water bottles can be used as screens and partitions.
Even old bicycles are not entirely worthless.