Id like to second Undercroft. I had a blast playing that. Also, if you are into old school Bards Tale-esque games you simply must check out The Quest. It is a HUGE game. Combined with all the expansions it offers an incredibly expansive fantasy world to explore, fight monsters and quest around. We are talking literally hundreds of hours worth of playing.
Also, be sure to check out Aralon. Its best described as the Iphone's Elder Scrolls game. Again, huge fantasy world with tons of content and quite pretty! Ravens Sword is it's spiritual successor. Its short and sweet but still fun.
Rimelands is interesting if you like experimental, turn-based gameplay (almost like the old hero quest boardgames) and Ash is a throwback to the Final Fantasy games (which are also available if you are into that). I'd recommend both.
Sacred Odyssey and Queens Crown are both action oriented. More like Zelda but with some (very) light RPG touches.
I'm not a big fan of Dungeon Hunter (1 or 2) but if you like Diablo chances are you will like them but Underworlds is a superiour game imho.