If you play tennis like we do, you likely have a car trunk or closet full of deflated tennis balls too worn out for rallying. Don't throw them all out...keep a couple around, as old tennis balls can become a useful household tool.
By cutting an "X" through a tennis ball and stick a broom stick through it, you've now created a wood floor scuff remover that erases black skid marks from no wax floors. Check it out, someone even applied for a patent for it.
I've also seen tennis balls with an x cut in them and then stuck over the ends of chair legs to prevent the floor getting scratched up. However that's not very pretty to look at.
Another good use is of the physical therapy kind...just lay on the floor and put that tennis ball right under where it's sore and ahhhhhh...hang out. Then remove the tennis ball and voila! Less pain!
Also great for flexing your shoeless feet...wiggle your toes over them, use them to find pressure points. Throw them in the dryer to fluff down comforters and jackets.
And walkers. :)
Just to touch on Simon's point of using them for laying on.
My mother was visiting and found a tennis ball at a nearby court to use for a massage, she rinsed of the ball and used it. The result was bright green stain on my rug.
So I suppose the moral is, don't use a wet tennis ball.