Diablo 2

Diablo 2 is a classic and very fun RPG from Blizzard Entertainment. Complete quests, find weapons, and kill demons! You can choose from a bunch of classes and level them up into advanced warriors. The higher you level up, the more skills and spells you can upgrade! Fill up your inventory with your best armor, weapons, and more!

 There is also a cooperative Multi-Player mode that is very fun!

You can buy the game on Blizzard's website: Blizzard Entertainment
If that's not your style there are torrents out there...