iOS: Previously mentioned app Sworkit now has a bigger brother with additional features. The original app is a great way to generate random workouts based on the amount of time you have and the muscle groups you want to target, but the pro version will talk you through it so you don't have to watch the screen to know what exercise to do next.
On top of that you can track your progress, access video demonstrations during your workout (if you need a quick reference), and try a new boot camp workout. Generally we like to recommend free versions over the pro versions, but Sworkit Pro is only $1 and the additional features are pretty helpful (namely the spoken exercise names). The one downside to the app in either case is that it's a bit slow (on the iPhone 4, at least), but it's nonetheless a very helpful tool for bringing variety to your workouts.
Sworkit Pro ($0.99) [iTunes App Store]