It probably varies based on states but in CA you can put a deposit in with the DMV in lieu of getting insurance. The deposit would be used for any damages you are liable for. I don't remember the amount but it made insurance seem like a better deal to me personally since the coverages went way beyond the deposit amount.

The requirement is "proof of financial responsibility" not "insurance" specifically. Every state allows you to establish a surety bond rather than insurance. If you've got $30k-$50k lying around doing nothing, you can let the state hold on to. So long as you don't get sued for damages related to your driving, you get it back when you stop driving.