Facade

Facade is a new type of gaming experience - in which the entire narrative is generated on the fly depending on user interaction. From the website:

Façade is an artificial intelligence-based art/research experiment in electronic narrative – an attempt to move beyond traditional branching or hyper-linked narrative to create a fully-realized, one-act interactive drama. Integrating an interdisciplinary set of artistic practices and artificial intelligence technologies, we have completed a five year collaboration to engineer a novel architecture for supporting emotional, interactive character behavior and drama-managed plot. Within this architecture we have built a dramatically interesting, real-time 3D virtual world inhabited by computer-controlled characters, in which the player experiences a story from a first-person perspective.

To me this seems like a very refreshing break from the way that narrative construction is typically handled in games. It also increases playablilty - by making a much more diverse set of possible outcomes, you encourage the user to play again and again.

Unfortunately, it's not available for Mac, yet, so I can't tell if it sucks or not. it is, however, free.