Buying a Car

Advice
RAC
(advice when buying a second-hand car)
DVLA
(detailed advice on how to check that the car you're buying isn't stolen)
AACarBuyer'sGuide
(just enter your requirements and it will list the makes/models you might want to consider)
Parker's - reviews
(straightforward, unbiased car reviews)

Price guide
Parker's - prices
(price guide for all makes, new & used.  Service is only free for cars made between 1995 & 2002)

Cars for sale
Exchange&Mart
(huge database of second-hand cars)
OneSwoop
(new & nearly new cars at discount prices)