This is a two-sided problem. People have extra space in their homes, many of whom would love to pay a little less on their mortgage. The other side is that people need extra space to store things. The rise of storage units is proof of this.
College campuses are known for not having enough parking. Nearby restaurants could open up their parking lots at slow times to gain additional income from an unused asset (their parking lot). They could easily adjust the hours that parking is available via an easy-to-use app or web solution.