NaK, or sodium-potassium alloy, an alloy of potassium (K) and sodium (Na), is usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with water and may catch fire when exposed to air, so must be handled with special precautions, but is not really explosive and not spreading radioactivity like water in reactors at accident cases.
Read more about NaK: Physical Properties, Synthesis and Production