I used to have VPI for our first greyhound. I did find that, for him, it was a good plan because he had a habit of hurting himself or getting sick in a major way about every 18 months, and every time the plan paid out so that I remained ahead of the game. But!! It would have been a losing proposition for the other three hounds we've had, since they've been more low-maintenance. If you think you have a pet that's likely to get some sort of cancer I think insurance is a good bet, but otherwise you may do better to just put the same amount of money that you'd pay in premiums into a special savings account - you'll likely find that that amount you have in savings will cover whatever expense your pet racks up at the vet.
And . . . (stepping up onto my soapbox) . . . the best insurance you can have if you have a dog is to brush their teeth every day. It's so very much cheaper to keep their teeth clean than it is to pay for veterinary dental cleanings, and dental disease is such a preventable illness, yet it can have a severe impact on your pet's overall health.
. . . stepping down now.
I have "pets best" insurance. For about 10 per month my doxie is covered for accidents and emergencies like foriegn bodies, breaks, etc. I can take her to any vet in any state that I want. Usually they re-emburse me for about 80% of the cost of the incident.
Theres are lots (dozens) of pet insurance companies out there. Someone posted a link to a rating source for a bunch of companies under a thread I started on "pet insurance for a rainy day". You might want to search for that thread it was very helpful.
I would like to add that the most common causes of death for dogs are cancer and kidney failure. The insurance I have is great about paying on claims and I would never want to own a dog without pet insurance. You never know, and the policy I have is the basic emergency and accident care, if nothing else it's good to have a basic plan.
If you are looking for insurance equine mortality or major medical, I would recommend The Hartford or Great American. They offer coverage for horses and other livestock. I used to sell equine insurance as well as farm and ranch policies. Hartford and Great American were the best IMO.
As far as pet insurance (I never sold these policies) I have heard from friends that they liked Veterinary Pet Insurance (VPI).
I used to carry mortality and medical on my horses and the minimum premium used to be $450.