Depends on how much you want to pay but there are two that I like to use both are in different price ranges. Mindbody.com and┬áhttp://www.fitsw.com/. Mindbody was very nice and professionally done – the only downside was that it’s pricey. FitSW seems more tailored for individual trainers / smaller gyms because its free – just not quite as polished as Mindbody but still very good. Both Fitsw and mindbody have excellent ways to track progress on your client’s fitness and they both do a great job at creating and recording workout plans and diet plans. They both are very helpful in helping me manage all my client’s info in one location.
At our Club, we utilize CSI Software. It has a more analytical base, and is designed well for mutli-site facilities. For a studio or facility specific to personal training, there is MindBody, which has a great user interface, is web-based, and makes it simple to book appointments online. Those are the two that I am most familiar with. Hope this helps!