Body by Science workout. It's based on the book by Doug McGuff (he's a doctor) and John Little. It uses exhaustive research to debunk a lot of the myths about exercise and what constitutes health. In the end they recommend high-intensity training through the Big 5 workout (that takes about 12 minutes once a week).
The basic idea is that you need to completely exhaust your body in order to call up the fat reserves for more energy and induce the building of muscle. As a survival tool, our bodies are designed to do the most amount of work while using the least amount of energy. We're great at adapting to be efficient, but that's not good for building muscle and losing weight. So the Big 5 workout pushes past efficiency to exhaustion, forcing our bodies to use up reserve energy. From a total muscle-energy deficit, the body is forced to replenish and rebuild, so instead of going to the easy-to-access food or muscle-tissue for energy, it goes to the more difficult fat tissue for the necessary energy.
When you've completely exhausted your muscles, your body takes between 5 and 10 days to return to normal energy and repair the mico-tears in your muscles (why you're sore). That's why you only do the workout once a week.
What's your experience with working out? What have you found that works for you?