Caverns of Xaskazien
Caverns of Xaskazien is an old-style tile-based roguelike that, although the graphics aren't anything to write home about, is surprisingly addictive. It makes good use of sound and is quite feature rich. There are few controls so it is easy to get started and a good choice for the Roguelike newbie but it will take a while to master. It is a 'semi-realtime' game - pause and the game will continue without you. An interesting feature that takes a little getting used to is that monsters fight you in the same square on screen - it looks like they've disappeared, but they're actually right on top of you.
Don't wake the sleeping dragons!