I've discovered a new drug: The Battle for Wesnoth. This is a Free Software game which is available for Windows, Linux, Mac, and many other platforms. It's a turn-based strategy game with both single-player and multi-player modes.
The gameplay is very simple: you have a bunch of soldier units which can move along the hexagonal grid. Your units have various characteristics, such as different kinds of attack and defense, magic, flight, speed, or other traits, and you must use these traits to defeat an enemy or capture a position on the map. There are other nuances, such as characters who work better in the day or in the night, and most importantly, characters who work better on different kinds of terrain. As your characters defeat enemies they acquire experience and advance to new ranks. You can recruit more units by getting gold, which you get when you occupy a village.
With these simple rules the gameplay is easy to pick up, even for a novice such as myself, but the games are quite entertaining. There are a number of single-player campaigns and there is also a multi-player mode.
What's most interesting about this is that this game is completely Free. You can download the code, artwork, and music, and modify or re-use it in other Free programs. This is rare in the world of games, especially for such a high-quality game.
The other day I was playing the game when I realized it was almost time to get up and go to work. That sort of thing hasn't happened to me in a long time and is a testament to the addictiveness of this game.