I've been using the "Fitness Online" app, and I really like it. I pick my focus for my workout series, and it tells me what to do, and it mixes up the muscle group. Right now I am doing a chest-centric workout, so I will do something like:
1) Arms
2) Back + Shoulders
3) Chest
4) Legs
5) Chest
I really like it. Half of the battle is always finding different exercises to target different muscle groups. This app does all of that for you.