The app/website I’ve used is the training notebook. It has allowed me to run sessions from anywhere, not needing to lug any papers around. I can sign up clients, have them sign waivers, and create programs. Pretty helpful.
http://www.FitSW.com really has helped improve my personal training sessions. It works across iPad and Android devices. My client’s also love being able to track their weights and exercises and it makes things a lot easier than using the yellow cards my gym used to use. It is a quick signup, completely free (I tried a few on the list but then they limit features and charge a lot to use the pro version). FitSW.com provides online tools like workout tracking & organizing, goal/body measurement progress tracking (in very nice easy to read graphs), diet plan creation, allows you to provide demo videos for your clients and includes a scheduling system. Its made my job much easier and you can’t beat the fact that it is Free.
Have you tried ChallengeLoop? http://www.challengeloop.com. The app allows you to motivate your clients to achieve their fitness goals and track their progress along the way. It is both an iPhone app and website.