Rental agencies already have insurance on their equipment as well as liability insurance. That doesn't get the customer off the hook though since the insurance carrier will try to recoup its costs from the rental customer if they can.
If you're using the equipment at home your homeowners policy will cover you but probably not the equipment itself.
When you're hauling rental equipment your personal auto coverage should cover you. Or as my insurance agent put it...."with personal auto coverage anything you pull is covered for liability, but if the vehicle is covered on a commercial policy anything you pull needs to be listed on your policy". So if you're using your company truck to pull a boat the boat isn't covered. I specifically asked about rental equipment. He said there shouldn't be a problem if I rented a tractor for a day, but if it was a long term lease (30 days or more???) I should add it to my policy or buy the insurance from the rental yard. This probably varies by state but thats how it is in IL (according to my agent).