I grew up on strategy games and starcraft (thus rule 34 proofing my childhood), but my first first person shooter was an open source project called sauerbraten. It looked terrible, with graphics that resembled an n64 game and a color palette that consisted of brilliant green, radioactive orange, and about 100 browns and blacks, but it had a strange allure to it's simplistic design and had a very good multiplayer scene. Anyway, after that I played Half Life 2, which blew my mind with what I had been missing out on.