In the late 80s I was addicted to a game called Dungeon Master for the Atari ST. It had incredibly sophisticated graphics for the time and totally compelling game play. It was a complete breakthrough, but as far as I knew I was the only one who thought so. Turns out I was wrong. Lots of people felt the same way. In fact there's an interesting article here about the importance of Dungeon Master in the evolution of computer gaming.
So I was really pleased to see that someone has recreated it in Java. Dungeon Master Java (link fixed) is free and almost identical to the original - maybe even better in some ways. The monster AI is much smarter and it comes with a level editor.
If you're a young pup who thinks Doom was ancient history, meet its granddaddy. If you're an old hand like me, enjoy a trip down memory lane!