Zombie Panic: Source. It's just so much FUN! I also remember being addicted to Uru and Riven (Both part of the Myst games) and Half-Life 2: Deathmatch. Tron 2.0 also had me for a while.
EDIT: When I was about 7 or 8, Runescape was my life. I finally quit when I was about 10- I have never played an MMORPG since.
EDIT: When I was about 7 or 8, Runescape was my life. I finally quit when I was about 10- I have never played an MMORPG since.