I did a 15 minute Tabata workout today. Until a week ago I had never heard of it. You take three exercises that utilize large muscle groups and do each exercise for 4 minutes. You need a timer (which you can download to your PC or ipod or mp3) and do intervals of exercise. 20 seconds of full out, fast as you can exercise and then you rest for 10 seconds. You do that for 4 minutes or 8 sets. Rest for one minute and then move on to the next exercise. Today I sprinted, push ups, and squats.
I am going to try it for a couple of weeks and see what happens.