1. Back Massager - Put two tennis balls in a tube sock and place them between your back and a wall. Lean against the tennis balls and massage into your tight shoulder and back muscles. You can also use a single tennis ball for your back, foot and legs.
2. Dryer Balls - Instead of commercial dryer balls, put 2 or 3 tennis balls in the dryer to help circulate the air better, which will dry your clothes more efficiently. They will also help fluff comforters or pillows when you dry them so that the filling won't clump up.
3. Scuff Removal - Here is a great tutorial from Ron Hazelton's House Calls, showing how to remove black marks off of floors.
4. Help With Parking - Hang a tennis ball from a string in your garage as a visual cue of where to stop so you don't hit the wall or your stored belongings. It works best if you measure and place it where it will hit the windshield or rear window. That way, you can't miss it!
5. Protective Homes for Field Mice - Don't laugh. In the UK, harvest mice are endangered and conservationists are using tennis balls from Wimbledon to protect the little critters from predators. Read more about it at BBC News.
(Images: Life Hackery)