Nike+ Training Club (Free, iOS and Android): Along with 85 custom-built workouts, there are also exclusive sessions from celebrities and pro athletes. Even better? Detailed instruction and audio support ensure that you use proper form.
FitnessFast ($1, iOS): The exercise library — categorized by body zone — is simple to navigate. You can also create custom exercises, add notes to your workouts, track your body statistics, and use a timer for your sets.