Insurance for theft, vandalism, fire, lightening, a wreck (drive into a lake, etc) and so on is relatively inexpensive, and is covered by some homeowners insurances. MAKE SURE your agent, preferably in writing, acknowledges that you've a real tractor, not just the "garden tractor" or "lawn tractor" sold at Sears and the like - the agent may balk when you show up with a claim for $20,000, not $2000. Once my agent realized the actual value of what I wanted insured, I had to purchase a $150 per year rider on my homeowners policy.
Now, the most comprehensive insurance, for liability and medical costs, is fairly expensive (about $1500-2000 for a compact tractor and implements in my area). Don't even consider loaning your tractor or letting someone else outside your family use it - if a neighbor / friend has some work to be done, and you feel so inclined, do the work yourself. If someone gets injured, even disabled, or, god forbid, killed, on your tractor - you are fully liable, and there's a lawyer around every corner who will take the case in order to try to liquidate all your assets. Don't go there.
