Beta blockers were a type of drug that slowed down a person's heartrate and pulse. A certain class of them could be used to slow down a pulse to the point that it would appear that someone was dead.
Metroprolol and atenolol were commonly available beta blockers that would slow down heartrate. Tetrodotoxin was a neurotoxin derived from a puffer fish that could cause the consumer to appear in a death-like state for days. (CTU Operations Manual)
At the end of Day 4, Jack Bauer used beta blockers to slow his heart rate down in order to convince Dale Spalding that he had been killed. ("Day 4: 6:00am-7:00am")