Out at the agility field today, I found a Boomer Ball (http://www.boomerball.com/) that someone left behind. Lexi loves to play with soccer balls, and this ball is supposed to be somewhat indestructible, and built specifically for dogs.
It is made of a durable plastic that is supposed to withstand chewing. But, I found that the plastic was too hard for a little dog. Lexi didn't seem to mind, but I was uncomfortable with her banging her head against it all the time as she herded it around the field. Also, I was afraid that it would hurt her if she ran in front of it when I kicked it to get it moving. But, she did love it, as she loves any big ball she can nose around.
All in all, I'd rather stick with a cheap $2 ball from Wal-Mart or a cheap soccer ball.