AutoSlash cuts the time it takes to find the best car rental rates. Enter your desired dates and location on AutoSlash.com, and the service scours the Web for discounts and coupon codes from companies such as Hertz and Avis. Within seconds, a list of the cheapest options pops up. If you reserve a car on the site, it automatically rebooks the reservation if a better deal comes up before your trip. Another plus: You can cancel the reservation at any time.

COST: Free.