Another FPS game, based on a heavily modified QuakeII engine. A classic in its day, and still played a lot (sort of) through CounterStrike. It was unique in that it had great single-player and multi-player modes. QuakeIII, for example, is basically multi-player only. You can play by yourself with "bots" but that's not how it's meant to be played. Historically, FPS games have had kinda crappy single-player modes so HalfLife was kinda unique for having great single-player and multi-player.