League Of Legends is nothing like Diablo.
Drakensang Online was a good suggestion. Theres also Fortune Online, Path of Exile in beta, Mythos is another good suggestion but i thought that one felt too grindy really early on.
Darkspore is another game similar to Diablo but isnt free to play, it has been on sale several times for $6 tho.
Try Hellgate, that is a really kickass fun game and it plays in a third person view like a shooting game but its very similar to Diablo in that you can just farm demons like crazy, its really cool, definitely check it out.
I forget what else right now.