Half-Life (1998) was really good already, but the sequel, although it is different from its predecessor, once again proved that Valve can do real wonders. Its graphics are amazing for the time the sequel came out (2004), actually, it really was a great year for videogames, up there with DOOM 3 and some others -- games from back there really did evolve video game technology, cimented their future. And Half-Life 2 one of their responsibles. Might be the foundation, once again, of how FPS would evolve.