Diablo III might be doing a good job of quenching your thirst for action RPGs today, but let's not forget its humble origins:
Rogue. Ahem, sorry, the first Diablo. You may rarely feel the urge to play the 90s classic, however, if you do in the future, you might want to check out Freeablo — a cross-platform rewrite of the game's engine that's in the works.
A user by the handle "wheybags" announced the project on Reddit a few days ago and linked to an, uh, official website and the GitHub repository where the rewrite's code resides. Yep, it's open source under GPL.
Now, this isn't the whole game — just the bits that put the data on the screen and allow you to play with it. You'll need to original files to do anything with Freeablo and even then it's barely playable.
The biggest issue I can see — other than the monumental task of reverse-engineering Diablo — is Blizzard itself. The company has a reputation for fiercely protection its brands, even chasing after things no sensible person would consider harmful.
I'm not sure if Blizzard would shut a project like this down, but I certainly wouldn't discount the possibility.