Travelsupermarket.com - lots of choice of hire cars and very competitive prices. Also has flights, hotels and other holiday related stuff!
Zest* - one of our favourite hire car companies with great customer service and really clear, easy to use site, packed full of all the information you need.
Holiday Extras* - great site dedicated to all those extra holiday things you might need to book such as car hire, airport parking, transfers, airport lounges to name but a few.
www.kayak.co.uk - clean, simple site for booking car hire as well as flights, hotels and more.
www.momondo.co.uk - compares the best offers on car hire, flights, hotels and package holidays.
www.expedia.co.uk - car hire, flights, hotels and plus some nice extras and a 'things to do' tabs with some great ideas.
www.opodo.co.uk* - clear and useful with online booking for car hire, flights, hotels and lots of handy extras.
Skyscanner* - despite the name it does car hire too!