Go get Nox (it's on GOG) - it's older than Diablo...or maybe not. They came out at about the same time but Nox was a bit overshadowed by Diablo, so it's not as popular. But it's very good, and it's not actually Diablo, although it may look like it. I've had lots of fun with it. One particularly interesting feature it had was that each character actually had a separate campaign. Sure the plot is the same, and they do cross some of the same places but it's a separate thing.
Sacred and/or Sacred 2 (they are on Steam and maybe GOG, too) - I've played the first one only and it was alright. The skill system was actually interesting but I can't really remember how it worked. I do remember you could build combos yourself, putting the different skills you had in sort of a macro - to be executed in order when you use it. And if I remember correctly, you don't have mana but everything had cooldowns. And it wasn't too bad. I certainly want to revisit it one of these days, maybe give Sacred 2 a go. It was amusing. EDIT: I should mention here that I remember the game wasn't that good. There were bugs and glitches and weirdness. The movement and attacks were a bit...off, too, from what I recall. I know it was partly because I played it very close after release (you had to use codes to even continue playing at one point) so some of it would have been fixed, but don't expect amazing quality game.
Path of Exile is free and it's quite good, too. The passive skill system gets some getting used to (looks a bit overwhelming [http://www.pathofexile.com/passive-skill-tree/] but stare at it for a while and it would start to make sense) and the active skills are implemented very interesting - you get skill gems (as quest rewards or drops) and you can put them on slots in your equipment and use them right away. If you cover the requirements, that is. There is little stopping you from having an Assassin with ice spells or a ranger with brute melee skills. The classes are just slightly more predisposed to some particular skills but you can mux and match anyway.
If you can find it, Blade & Sword is...interesting. It's a Diablo clone put in an Asian setting and...well, I can't remember much. It had some nifty ideas and it was also largely Diablo, too. I recall being very frustrated, though. If you do find it, word of advice - grind more. I went through most of the game just by clearing all areas but I was quite underlevelled (and I think you can't go back after you go through an act). I finally just stopped playing right at the friggin' last boss. There was a large stretch of land filled with monstrosities I had to cross and not get hit to even have a shot at beating the boss. Which was annoying, hard and finally made me quit. But looking back, it did have its moments.
Fate, I think you'll like a lot. Not sure where to find it, though. I had free access (for limited plays) to it along with few other games when I bought my laptop, so that's how I managed to try it. It's similar to Torchlight in that it's very much like Diablo 2. Well...how should I put it - I even stopped playing because it was too much like Diablo 2. Quite a lot like Diablo 2. In fact, I think one of the people from the D2 team worked on it, similar to how Torchlight came about.
EDIT: I found this [http://www.giantbomb.com/profile/wastelanderone/lists/ginormous-list-of-diablo-clones/74701] list, which I thought was interesting.
