Came across Adzuna cars when hunting for a car, wouldn't use any other car classifieds website since finding it. Saves me having to look across lots of different sites as they have ads from all the main sites.
I was searching for a Ford Fiesta and they have lots of graphs to show what I should be paying for cars from different years. The map view was also really useful as I didn't want to travel far, so could view cars only in North London.
The other handy feature was being able to sign-up for alerts to new cars added, so I didn't have to keep checking the site. Plus there's a fun car quiz to do too!