I got the idea for 15/15 and 30/30 workouts from Jon Hinds at Monkey Bar Gym. It's a simple but effective concept: 15 (or 30) seconds on, 15 (or 30 seconds) off. For example, say you were going to do a 15/15 workout of single kettlebell Clean and Presses (C&Ps). You would set an interval timer to go off every 15 seconds for a set amount of time, let's say eight minutes. The workout would begin with you doing C&Ps for 15 seconds on one side. Then you would rest for 15 seconds, before doing 15 seconds of C&Ps on the other side. This would continue for the duration of the workout... work, rest, work, rest, etc. The same goes for 30/30s except the duration of rest and work intervals increases to 30 seconds.
The possible workouts that can be built around 15/15 and 30/30 intervals are endless. One way to really get a lot of bang for your buck is to do opposing exercises. For example, you could do a 15/15 built around kettlebell Swings and Squats: Swing 15-sec, rest 15-sec, Squat 15-sec, rest 15-sec, and so on. Or you could do bodyweight stuff, alternating between push-ups, rest, and chin-ups.
15/15s and 30/30s are a fun and effective way to combine strength training with fat burning. Best of all, they are easily customized to an individual's needs and preferences. Give it a try!