Hi there! There are a lot of options out there. I think it really depends on how much you want to pay but there are two that I have used in different price ranges. Mindbody.com and┬áhttp://www.fitsw.com/ are my favorites. 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.
They both do a great job at creating, recording and sharing workout plans and also diet plans. http://www.fitsw.com/ also is excellent for tracking progress on client fitness. They both are very helpful in helping me manage all my client’s info in one location. Good Luck!