old and broken motor mounts will cause the car to vibrate heavily at idle, and more so if you're an automatic. running the a/c at the same time can contribute as well.

like the other guys said, check your plugs and replace if needed, how long have the plug wires been on the car? check the dist. cap and rotor and replace the fuel filter if it hasn't beenn done since you've had the car or don't know when it was last replaced.