I use fitnessbuilder.com Its a website, but also an app, so I can use it anywhere.
I like the overall functionality. Its very easy to write, email and print programs. It has a lot of exercises, programs and the ability to photograph, write a description and add your own exercises. You can also invite your clients to join (for a fee) and you can track their progress. I like that I track my clients progress myself during our sessions and then make notes right on the session or on the clients profile.
The downside is that I still find numberous exercises that are not listed, that I then have to add myself. I wish there were more baseline stats that you could add, like a mile test, squat test, push-up test, etc. I also wish that there were more modifications for certain exercises listed.