I'm gonna slink back into the shadows before I get yelled at for not disapproving that their legs aren't backwards unlike http://upload.wikimedia.org/wikipedia/commons/f/ff/KomodoDragonRinca1.jpg.
No, I'mma yell at you for something different.
Again with the backwards thing...are people really that grossly uneducated? Digitigrade legs are NOT backwards knees, those are ankles. Screw it, I'm just copy-pasting the Wikipedia article now:
A digitigrade is an animal that stands or walks on its digits, or toes. Digitigrades include walking birds (what many assume to be bird knees are actually ankles), cats, dogs, and most other mammals, but not humans, bears, and a few others (cf. plantigrade, unguligrade). Digitigrades are generally quicker and move more quietly than other mammals.
While humans usually walk with the soles of their feet on the ground, i.e. plantigrade locomotion, digitigrade animals walk on their distal and intermediate phalanges. Digitigrade locomotion is responsible for the distinctive hooked shape of dog legs.
There are anatomical differences between a plantigrade and digitigrade limb. Digitigrade animals have relatively long carpals and tarsals, and the bones which would correspond to the human ankle are thus set much higher in the limb than in a human. This effectively lengthens the foot, so much so that a digitigrade animal's "hands" and "feet" are often thought to correspond only to what would be the bones of the human finger or toe.
Unguligrade animals, such as horses and cattle, walk only on the distal-most tips of their digits, while in digitigrade animals, more than one segment of the digit makes contact with the ground, either directly (as in birds) or via paw-pads (as in dogs).
Edit: Seems someone brought it up already, but I'mma leave this here for the other people who can't be arsed to read the whole topic. Education, ftw.
Edit #2: Also, have some of you
never seen an ostrich? Digitigrade bipeds DO EXIST, you know.