If it has a choke that is the most likely culprit. If it is fuel injected, it's a sensor.

I'd go with what Kendall said here as the most likely scenario. The fact that it improves as it warms up suggests that this is very likely.

A couple more ideas that haven't been mentioned yet, all of which have caused me problems from time to time -

I had intermittent problems with an '82 Holden which used to stall at lights. It turned out to be the ignition module.

I don't think anyone has mentioned the fuel pump yet, or the fuel pump relay. Have they been checked?

Most recently I had a problem with my car (a Ford Fairmont) running rough when it was idling but smoothly when driving along. A whole series of things were replaced or tried - the oxygen sensor, the map sensor, cleaning the fuel injectors. What it finally turned out to be was a faulty coil - nearly new but obviously faulty. There was a weak spark, which affected the idling but didn't matter once the motor was running above a certain number of revs.

One of the most frustrating things with cars is when the problem is intermittent. Murphy's Law dictates that the problem will never manifest itself when the mechanic is looking at it, only when you are by yourself at home or stuck on the road somewhere. THEY KNOW!!

Good luck with it!