Outside unit runs continuously, even when system is off.

On A/C units, there is a low voltage line that leads to a contactor, which engages to close the high voltage circuit to the condensor unit. Over time, the accual contacts in the contactor can weld themselves together. Try shutting off your furnace breaker but leaving on the one for the outside condensor unit. If it still runs, your contactor probably needs replaced. Contact a licensed electrician or HVAC contractor and tell them you think your contactor is in need of replacement. Is a fairly simple and quick replace, but the contactor itself is not cheap.