There are over 20,000 car rental locations in the US. There are several large brands that dominate the market. Enterprise, which also owns Alamo and National, is the largest with over a million cars. Hertz is 2nd. They also own Dollar and Thrifty. Avis and Budget merged and are now the 3rd largest.
There are many smaller and independent car rental places. They often specialize in exotic and luxury cars that the large brands don't carry.
There are many ways to get discounts on car rentals. Most offer discounts to members of AAA, club stores like Costco, and many other organizations.
In most cities, rentals are less expensive on weekends because of higher demand from business travelers during the week. Summer is more popular for tourists. Rentals are less expensive during the off season. Renting for more days usually reduces the price per day.
Airport locations have the largest inventory and turnover. You can usually find better deals at the airport than other smaller locations. Many airlines let you book flights, hotels, and car rentals together. These bundles cost less than booking separately. Booking ahead of time is better than showing up without a reservation.
There's always paperwork and extra fees when you get to the counter. Various local taxes appear even if you paid in advance. Some rentals come with mileage limits, so watch out for per mile fees.
Every company will try to sell you insurance. You may already have coverage through your personal car insurance or credit card benefits. Damage to your car can be expensive, so note any existing damage before leaving the lot.
In areas with toll roads and bridges your car will likely have a transponder for paying tolls. You will be charged for tolls that accrue.
The most unpleasant fee of all is when you don't return the car with the same amount of fuel. Car rental companies charge far above gas station prices. Find out where there's a nearby gas station and allow time to refuel before returning the car.