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!
The best application or software really depends on you and what problem you are trying to solve.
Personally I use Google Calendar and Square for my client scheduling and billing as they integrate nicely with my various devices.
If you are interested in showing value through client progress, especially when it comes time for a resign, you should check out http://trainermetrics.com . It’s simply a fitness assessment and client progress tracking service that allows you to print reports.
I hope this helps!