I only run when chased, so biking has always been my preferred method for a cardio workout. However, I've never been much for stationary bikes or spinning. The RealRyder ABF8 spin bike, however, might change my mind. (Though I doubt I'll have the $1,995 to buy one anytime soon.)
Instead of a rigid ride, the bike has an articulating frame letting you lean, steer, and sway from side to side as if you were on a typical road bike.
The company claims the instability allows for a more complete workout, engaging core and upper-body muscles since you are in fact trying to keep your balance.
If you haven't already, watch the video to see what I mean. (Be warned, though, Feist is playing throughout the 3-minute-long video.)