Collision Insurance

This protects you against damage to the car you are renting.  It ought to be quite inexpensive to buy, but rental agencies often charge $15-$25 per day for such coverage, usually called either Loss or Collision Damage Waiver (LDW or CDW, resp.) Some major credit cards provide backup, limited collision coverage that is secondary to your personal insurance if your auto policy covers rentals. However, there are some premium or business-type credit cards that provide primary coverage that may be included in your annual premium or purchased separately.  For  comparison, if you have an American Express Card, you may be able to purchase extensive primary coverage at a total cost of under $25 for a rental up to six weeks.  Finally, there are numerous travel insurance companies that sell CDW insurance online, such as total travel insuranceTrip Insurance StoreTravel Insurance Center at a cost well below that of rental car agencies. 

For further information about renting a car, there are many online references, including Best Car Rental TipsTravel SenseThere Are Places.