Squat Til You Drop

Unless you have loads of time on your hands, when you need to work out it had better be quick and effective.
Ladies and gents:

Squat.

Squats work all of the muscles in your legs simultaneously, giving you more bang for your buck.  Because you're bearing your whole weight while you perform the squat, you're also burning calories.

Studies have been done to dispel the convincing myth that in order to lose fat you have to do hours upon hours of cardiovascular training (i.e. running, cycling, swimming).
So if you don't have hours upon hours (really, who does?) then just take a few minutes and do this squat challenge:

The song is slightly annoying...the instructor is slightly annoying...but the 3 minutes and 13 seconds of squatting will tone your legs, burn calories, build muscle, and all those other wonderful things that keep your body in good health and good shape.

If you want to be a nerd about it, you can even calculate a close estimate of how many calories you burn while you're squatting.