Potassium cyanide - this bottle contains enough to kill around 1000 people. But the 'cyanide' that many of is would have heard/read about is actually hydrogen cyanide (HCN), a gas at room temperature.
Cyanide binds to hemoglobin once it enters the bloodstream, although it doesn't work in the same way carbon monoxide does (by displacing oxygen completely). Instead, cyanide travels throughout the body and inhibits cellular respiration by blocking the final step of the electron transport chain, preventing the reduction of O2 to H2O. Stopping this step means the membrane potential required to generate ATP (in the mitochondria of cells) is never reached.
Since ATP is essential for normal cellular processes, depriving cells of it is no good as it means we literally lose all function in the body (including the nerves and heart). Furthermore, cells are forced into anaerobic respiration, leading to a buildup of lactic acid and ensuring a painful few minutes of acidosis before death... Hope you learnt something new today!