If renting at an airport, I suggest Hotwire or Priceline. Hotwire is quite a bit easier to use for car rentals.

If renting not at an airport, I use carrentals.com. This usually points me to Enterprise anyway.

One thing to be careful about, Enterprise is often closed from Saturday noon to Monday morning. So let's say you want to rent from Saturday at 1PM to the following Sunday at 1PM. Carrentals.com won't look at Enterprise since they aren't open. However, it's possible it could be cheaper to rent at Enterprise and pick it up at 11AM on Saturday and return it on Sunday when they'll be closed but you just pay for the extra day until Monday.

I find Enterprise to be less high-pressure on selling add-ons too. Where I live, my other option is Budget, and they are always trying to sell gas, insurance, toll transponder, GPS, satellite radio, roadside assistance.

That reminds me, be a little careful as some locations will sell you roadside assistance without asking. Happened to me with a deep discount vendor at LAS airport. My reservation had been for $168 but the receipt was for $203, and it turned out they automatically added $4.99 a day for roadside assistance.

Also, I think Budget and maybe some others automatically charge a fixed refueling fee if you drive less than a certain distance. With Budget I think it's $13.99 if you drive less than 75 miles. If you don't want to pay this, you'll have to tell them. If you rent at an airport, always keep your gas receipts. If renting from Enterprise, you usually just return with the tank as full as you left so they can tell from the gas gauge if you bought enough gas.