nethack is a classic (I hear)
I recently have dove back into the mines of Moria with umoria 5.2 for PC. Man this game is fun, not really an RPG though, more of a dungeon-crawl game. I like how you can do anything, like tunnel through walls, teleport, make walls, clone monsters, very cool. And a good challenege, it is hard to kill the Balrog.
My latest power-gamer stunt is I have this chamber with a young red dragon in it, and I have a ranger with 3 wand of clone monster. I go to the door of the room, clone a dragon, lure it back to my killing fields while the original always seems to stay in it's pen, then i kill it, get mad xp and hopefully get stat potions since I am on level 25. When the wands run out of charges, I recharge them and repeat. This is what I have been doing with my spare time for the past 3 days.