Nike Training Club is a great (free) app for if you want a workout regime which allows you to control when and what you do. Nike positions it as your own personal fitness trainer.
The app is divided into 4 goals: Get LEAN (high interval cardio drills), Get TONED (light wieghts and intervals), Get STRONG (increased weights and reps) and Get FOCUSSED (15 min targeted workouts). Each goal features a beginner, intermediate and advanced level and a number of different workouts (with engaging titles such as "crunch +burn" or "Hurricane"), with different exercise combinations and duarions. It really couldn't be simpler (even I could work it out).
Each workout is divided into a warm up, exercise section and stretch. A voice guides through the workout, which you can set to your own music, and each exercise also has a mini video to show you how to do the exercise. At the end of each work out you get a workout summary, and you can track your progress and earn rewards.
The good thing about this training system is that you can really do it any time, anywhere and since the exercises are set according to fitness level, you can be guaranteed to push yourself without overdoing it. It takes a bit of time to pick up the exercises if you are unfamiliar with them, which can mean a bit of stopping and starting at the beginning to get the hang of them. You can also easily replace an exercise with something different if it doesn't work for you.
For spring Nike has released updates with sessions with celebrities like Lea Michele and professional athletes such as Shawn Johnson as well as some new workouts.
If you prefer to follow a work out or have regular motivation then this may not be for you as it really is all about you burning through exercises and developing a personal record. But if you enjoy getting focussed and sweating for 30 or 45 minutes then you will really enjoy this club!