Here in CT, you can drive your tractor on public, non-limited access roads with no registration if you're going from one field you're working to another and to your farm. BUT, you need to have a working farm. Unfortunately, that doesn't mean you can qualify if you've got two horses and a cow; it means you've registered with the Department of Revenue Services (DRS) as a commercial farm, applied for a Sales Tax Exemption Permit, and generate revenues equal to or greater than $2,500 per year from operating the farm. If you meet these requirements, the DRS will issue you farm plates. In addition, you MUST have an SMV sign on your tractor. If it's not visible because you have a rear implement there (e.g. hay wagon, baler, etc.), that attachment must have a visible SMV sign also. It's also highly recommended that you turn on your headlights and 4-way flashers if so equipped. In addition, you need a farm insurance policy to insure you for liability while on a public road. Most homeowner's policies only cover you while on your own personal property.
If you can't conform to the above requirements (like if you don't have a working farm), you always have the option of registering the tractor as a commercial motor vehicle, just as you do with your truck. That's about a $90/year cost. Of course, your homeowner's insurance doesn't cover you for damage or liability, so you need to add your tractor to your car insurance policy for some additional fee. If we're helping a neighbor move rocks, loam, or some such, I don't think we'd be covered for that via the farm or the automotive policy because we're now doing construction -- so you'd need to check that with your insurance company.
I spoke with our resident state trooper last week and he gave similar advice to that of redsox1, namely, it's in the officer's discretion. He indicated that few police officers will bother a responsible citizen driving properly and being reasonably cautious, especially for short distances from the tractor owner's property.
No matter which way you go, what bothers me most is the liability exposure we have. I'm a businessman and I've seen too many times how the best of intentions in helping a friend, family member, or neighbor can sometimes turn into a nightmare because of a freak accident or digging up a cable and such. So, I cringe when I think of being exposed to thousands, tens of thousands, or more dollars of risk. Without the protection of a corporation or LLC, it's our actual homes and investments that are exposed to a lawsuit -- scary!
The foregoing hasn't stopped me from helping a neighbor, but it shore makes me enjoy it less and keeps me within a few houses of my property. For what it's worth, our local trooper said the specifics of the law vary by state, but most were quite similar.
I hope this helps.