GOG recently added to the catalog, not a Diablo clone but it can serve as a substitute to scratch that action RPG itch. If branching out into action adventure territory is of interest, it's worth checking out [url=http://www.gog.com/game/silver]Silver and the Ecstatica series. Silver has aged better than Ecastatica 1 +2, those games can be quite aggravating. Re-spawning enemies and old school controls, non-stop fighting but practically no RPG elements. But the atmosphere is great, especially Ecstatica 2 is quite dark despite the cartoony style.

For a straight up shameless Diablo clone, I recommend checking out Blade & Sword, pretty much Diablo but with a Chinese theme, made by a Chinese company. Smooth gameplay, virtually no learning curve for Diablo players. Only has 3 character classes: A quick female assassin, a slow but heavy warrior and a standard "allround swordsman hero" type.
Skill trees are simpler than in Diablo and self-explanatory. The only immediate difference to Diablo is that you can hold down a key to block attacks so you can bunker up and once the enemies made their attacks you can quickly attack yourself. Other than that it's straight up Diablo gameplay with run-of-the-mill simple quests, potions shop, bosses etc. Loot system isn't as good as Diablo but that's to be expected. I didn't finish the game with any of the 3 characters. it became too boring too quickly and the enemies have a lot of HP on normal and hard difficulty. I recommend playing on easy because otherwise it's a waste of time, the game isn't overly difficult but it takes ages to smash a simple zombie and that's just too much grind. Even on easy the monsters have more health than they have in Diablo at the beginning.

Gameplay video:
https://www.youtube.com/watch?v=hwFoDzu5-S4