Check out flOw, a beautiful intuitive game set in "an abstract aquatic world inviting the player to dive in, to learn, to explore, to survive..." There is little instruction before you start, you just figure it out as you go in a well-designed learning curve.
The game is based on the theory of flow by psychologist Mihaly Csikszentmihalyi, which describes the state of focused and rewarding, almost Zen-like immersion in an activity. Achieving a state of flow depends on a balance between the challenge posed by the task (e.g. working, learning, gaming) and the performer's skill.
See also this old Wired interview with Csikszentmihalyi on the flow of websites.