Riding a bike is definitely a good source of exercise, although it does tend to work out the same muscles in the same fashion, over and over. In an effort to remedy that, the Caron Bicycle was created. It can be pedaled in six different ways, all of which still move the thing forward.
The Caron is mostly a pretty normal bike, with the big difference lying in its crankset/bottom bracket. In fact, if you've got the right type of bottom bracket shell, you can just buy that bit by itself and install it on your own bike.
Along with the regular pedaling motion that we're all used to, the Caron can also be pedaled with one foot (left or right) moving up and down at the front; one foot moving in full revolutions while the other sits at the bottom; both feet moving up and down; both feet moving in full revolutions, side-by-side; and left then right feet alternately moving up and down.
A bar-mounted thumb switch is used to select between the different pedaling modes, all of which are demonstrated in the video at the bottom of the page.
The designers of the Caron Bicycle are currently raising production funds, on Indiegogo. A pledge of US$245 will get you the add-on kit, while $570 is required for a kid's bike and $795 is needed for an adult model ... assuming all goes according to plans.
Should you be looking for a system that lets you move forward by pedaling backwards – or forwards – check out biXe Gear.