I saw something a little while ago about a photographer who rented out his equipment when he wasn't using it. He made quite a bit of money from it and it's apparently all insured through the company/website that runs it all.
I can't remember the name of the company/website but I think I saw it on the TV but he was saying that so far, he's never had a problem.
Personally I don't think I could do it, but I'm a hobbyist and look after my gear. Maybe for a pro who uses his gear as a tool and not to bothered if it has a scratch here or there it could be a lucrative sideline.