NetHack is a twenty-year-old free roguelike that's still actively played and updated. It's as complex as it is fun. This time, I'm not even going to attempt to explain it, because this article (http://nullprogram.com/blog/2008/09/) does it much better than I could. Tvtropes also has a great article: http://tvtropes.org/pmwiki/pmwiki.php/Main/NetHack
To play it, you'll need to telnet to nethack.alt.org. It's probably best to download a Telnet client, since the ones that come with Windows aren't very good. Putty works nicely for me.