Most rental car companies (Budget, Enterprise, Thrifty, etc) require a deposit which is held against your credit/debit card. It's typically >=$200, but they usually require a larger deposit if you use a debit card. I've been required to put down $400 in the past. They often don't make this deposit obvious, but you will definitely see it reflected in your card's account activity.

Would genuinely love to hear about companies that don't require this deposit.