runescape
Runescape is a MMORPG (Massive Online Role Playing Game) that started a few years ago as a simple, 2-d Java-based game that was F2P (free to play).  Since then, it has grown to millions of users with the option of P2P (pay to play).  

Here is the common misconception: Runescape DOES NOT ruin your life.  Yes, while it may be addicting, the only thing that can ruin your life is YOUR actions and the way YOU perceive things.  Anyone that says "Runescape ruined my life" is a liar.  They ruined their own life by allowing themselves to become addicted to it.  Take my advice before you start to play Runescape, please. 
Bob: "Hey Mike, don't play that game Runescape, it will consume your soul and ruin your life"
Mike: "A game cannot ruin you life, only your own actions can"
Bob: Waaaaaaaaaa
Mike: "STFU, n00b!"