Terraria.org

I love Terraria for so many reasons they probably wouldn't fit on this page so I'll keep it short. It runs on weak machines so you don't have to be a hardcore rich gamer to play it. There's a huge amount in customization - how your characters looks, how you play, how your home looks. Multiplayer is a lot of fun. It starts off really simple but over time as you learn more about the game it gets more difficult but also more interesting and exciting. The creative possibilities are endless and I can't stop playing it!