This is a tip that probably only applies to people who live in big cities, but if you do, and you're not already making use of it, read on. There's serious money to be made here. Increasingly, city dwellers don't drive anywhere. The roads are too congested, and it's easier to walk, take a bike, or use public transport. That means they don't have a car, and the car parking space that came with their property is going unused.
If you live in an urban area where parking is expensive and hard to find, you have a license to print money. There are a number of websites you can advertise your parking space to, some of which might even be local to you and specialized. You could be exactly what a wealthy but frustrated commuter has been looking for. Depending on your area, your space could be worth hundreds of dollars a month.