There's quite a few - I've had BMW's hat were really quiet, but the quietest car I've had was a Lexus GS 400 - I had to look at the dash to make sure that the engine was still running.
hybrids like the toyota prius, when sitting in traffic or driving very slowly they dont even have the engine running, all you can hear is the brakes as the wheels are spinning.
My Toyota corolla had a super quiet engine, I often tried to start it when it was running as I couldn't hear it. Also people crossing the road would jump to see a car so close before they heard it...... (is this a good thing ?)