Looking through these posts, I cannot believe no one has mentioned the granddaddy of them all (as a topic) - Quake 1!
I was always in to PC games, but only in 1997 when I started doing "Computer Science" at a school outside of my own (due to our computer lab being somewhat not great) was I introduced to the madness that was multiplayer Quake. Our really cool lecturer would let us play LAN games once the lesson was done on the shiny new PCs they had.
Having never used a mouse as a control scheme before, I played purely on the keyboard and was surprisingly good at it, winning matches against people who had been playing longer than I (and with the mouse!). Yes, I was a keyboard turner.
Then, I decided to use the mouse and thats when people would start swearing I eventually brought Quake over to our PC Lab (which had since then seen an upgrade) and spent hours after school fragging my friends (DM5 and DM6 ftw!). Then came all the custom maps to the game with all the mods.
Still one of my favourite games and pretty much unbeatable in terms of fun. Rocket jumps, trick grenade shots and a friggin nail gun. Thank you John Carmack for the memories