Mayhem Intergalactic is a game with simple rules giving rise to rich strategy. Each session lasts only 5-15 minutes, yet there is enough depth and replayability to keep you occupied for hours at a time. In a nutshell, it's a turn-based 4X-like strategy game, providing a streamlined version of the gameplay found in other turn-based strategy games. There is no resource micromanagement, allowing you to concentrate on strategy and tactics instead. Also, there's an alien wearing a monocle.
Screenshot of Mayhem Intergalactic in action.