I usually check www.carrentals.com to do a quick check of whats available and pricing, but I usually end up booking on the actual rental car website (ie: Alamo, Budget,Dollar etc.) so that I can try/use different discount coupons/codes
carrentals.com will also monitor your particular request and send you an email if there is a price drop (kind of a nice feature)
I would also recommend the Mousesavers website which updates current codes and coupons pretty regularly (by rental car company). You also might consider subscribing to the Mousesavers monthly email newsletter, too. Both have helped me out numerous times on finding decent car rentals.