One key area for this is video gaming. The Nintendo Wii system has been revolutionary, in that it has opened up motion gaming and movement-based gaming to video gamers. Although games like Dance Dance Revolution (which has the player literally dancing on a dance pad) have existed for some time, Nintendo has actually brought out a Wii game called Wii Fit, which works with a balance board. Through this game, users can actually record their body mass index, and all the game modes are in fact fun exercises to help the player stay fit. It has been a top seller, and will no doubt be imitated by other companies. Microsoft and Sony have since come out with motion systems for their own gaming consoles, so it is likely we may see some fitness-gaming on the X-Box 360 and Playstation 3 as well!
Of course, there are other ways to get fit while enjoying home entertainment. One of the obvious ones is simply to workout while watching television, using weights or an exercise bike while you enjoy your shows or movies. One way to motivate yourself might be to take breaks during the commercials, and then make sure to start up again during the show and workout continuously.