The major rental car companies in the US, with each company's primary market in parentheses:
Hertz Global Holdings: Hertz (business) - Advantage (downmarket/leisure)
Avis Budget Group: Avis (business) - Budget (downmarket/leisure)
Enterprise Holdings: Enterprise (local/insurance replacement) National(business), Alamo (leisure)
Dollar-Thrifty Automotive Group**: Dollar (leisure) Thrifty (business)
Independents:
-ACE Rent a Car
-Payless Car Rental
-Rent-a-Wreck (downmarket)
**Dollar-Thrifty has takeover bids by both Hertz and Avis Budget, so they will likely be absorbed into one of the two in the near to mid future.