In the event that you're looking for a new action RPG to keep you occupied until Diablo III's May 15th release date, GameSpy has profiled what they feel are the 10 best action RPGs released since Diablo II. I can't say I agree entirely with their list (where are Baldur's Gate: Dark Alliance and Champions of Norrath?), but there are a few games on there that definitely deserve the recognition:
Divine Divinity - 2002 I also think that Nox and Darkstone deserve praise, even though they technically came out right before Diablo II.
Divine Divinity managed to splice Diablo's hack 'n' slash DNA with the more complex aspects of RPGs like Baldur's Gate and a refreshing dab of humor -- something most fantasy worlds fail to invent while constantly fretting about Ends of Days and Chosen Ones. Skill trees, for instance, are shared among all character types, so hybrids of all shapes and sizes are viable if you know what you're doing. (And if you don't, well, you probably made the mistake of playing a Summoner.) The world, meanwhile, is impressively non-linear, and most NPCs react realistically when faced with a blade to the face. That is to say, by dying. Unfortunately, however, there is NPC clone of the person who decided to call the game Divine Divinity.
Titan Quest - 2006
So far, every other game on this list has gleefully slain its way through some randomly generated fusion of Blizzard and J.R.R. Tolkien's imaginations, which is kind of impressive -- but not really in a good way. So let's say you're looking for an exotic vacation spot for your olde-timey mythological take on Hoarders. Well, why not Greece? Minotaurs, Gorgon sisters, and of course Titans round out Titan Quest's roster of thematically appropriate loot pinatas, and its skill progression lets players pick and choose between all sorts of varied trees. No, Titan Quest didn't kick off a new generation of action-RPGs, but it did manage to all at once spirit the genre away to a new setting and capture Diablo's lightning-in-a-bottle feel better than most of its competitors.