I've had my iPhone 4 for about a month now and have got a good collection of apps going now. Some of my favourites are:
5K Runner Free.
This has an 8 week plan to get you off the couch and out running until it builds up to a 5k run. The programme is ideal for beginners and great to get you out and about. You can use it while running outdoors or at the gym and the audio prompts just help to keep on track and making progress every day.
Boot camp lolo
This allows you to select your current fitness level from beginner, intermediate or advanced. Next you select your equipment that you have to hand such as a chair or weights or a bench, you can then set the level on intensity that you want to work out to for that day. The app will create a work out routine for you to follow for that day and show a video of how to perform each exercise. It usually recommends things like jogging on the spot for 5 minutes, push ups or sit ups. I find it very handy for days when I'm at home and feel like a quick work out but can't think what to do.
Feel free Butt, Leg and Push ups
These are handy if you want to focus on one particular area of your body. Again, I don't really follow the set daily programme but use these as inspiration when I have the time at home. I personally prefer the Boot Camp app as it mixes things up a bit more but these are handy for those days that you just think - hey, my jeans are a bit tight today! and you can just focus on that butt area.