If you own a car, your own insurance covers it, no problem, done.
But if not, there seems to be no decent options besides the per-day insurance from the rental company.
Credit cards will cover damage to the car, but not liability. Even American Express's premium car rental insurance (at $25 per rental) that takes over from your own car insurance, doesn't provide any liability coverage. So if you hit someone else, you're fucked.
An umbrella policy won't cover it since they expect you to have car insurance for your own vehicle.
I tried to get a quote for non-owner car insurance from the company that does my motorcycle insurance and it didn't make sense at all. They would only only cover liability, not anything with the car itself or passengers...and the price was way too high for that limited coverage.
Just doesn't make sense...at the rental place, the collision coverage costs more per day than the liability coverage...yet I can rely on several cheap alternatives to avoid the CDW. Why doesn't Amex's premium insurance include liability?