Created by California-based Beach Body Fitness, known for their tough-but-effective fitness videos, this workout lives up to its name. It's a 45-minute workout that has intervals of four to five minutes and super-short breath-catching breaks. The intensity of the program helps to build core strength, and all-body fitness, which, the company says, can be attained in just two months.
Best for: You should be in decent shape to attempt this workout. It uses your own body weight to build muscle. It might be tough if you're carrying way too many pounds, so you might want to do some traditional gym workouts for some time before beginning. (The name is "Insanity," after all.)