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.
I personaly am a fan of the LiveStrong Calorie Counter, I think they are a solid company with great technilogical advances!
There were some great Apps suggested in this question as well http://www.ideafit.com/answers/what-is-the-best-foodnutrition-tracking-a...
The Nike+ App on iPhone is excellent! It is a great motivator for clients because they can keep track of the miles/kilometers while walking or running. Goal setting and success tracking is easy. It requires the Nike+ sensor in the Nike shoes.
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.