Sworkit: Think of it as an iPod shuffle for exercise. You choose your workout time (up to one hour) and your goal (full body, core, back, etc.) and Sworkit then generates a series of 30-second routines. No equipment required, so you can do them pretty much anywhere. There’s handy photos, too. At the end of every 30 seconds, a whistle blows and you’re on to the next move.
Free at the iTunes Store and Android Marketplace and it also works on your computer.