Counter-Strike is easily the best Mod for Half-life. It breaks the players up into teams of terrorists or counter-terrorists. Players earn money by killing their enemies and winning rounds. Each side has specific objectives that they each try and accomplish. At the beginning of each new round, they can buy new guns, ammo, grenades, armor, and other items with the money they earn. If you'd like more information go to the Counter-Strike webpage here or the CS Nation website here.