Current car - MR2 (Mark 2) - Owned for 18 months, intending to sell it in the next couple of weeks. I'm getting rid of it because I'm fed up of it really, all my driving is commuting or in the city.
Previous car - Astra (Mark 4) - Also owned for about 18 months, I got rid of it because I wanted something faster and more fun. I was at uni at the time and didn't need a car as I got the bus in most days. I know it has a bad reputation around here but honestly I couldn't fault the car.
The car I've owned the longest was my first car, a Punto. The main reason for that was insurance, after my second year of driving I could afford insurance for something bigger (ie the Astra)