I use Trainerize.com as well. Very user friendly! Easy for you and the client to understand. You can create workout templates and individual workouts. Clients can track and record their workouts with a calendar thats provided for them. Price is also reasonable.
I have been using trainerize.com. You can include video clips of exercises along with instructions. Your clients get their routines emailed to them and they can track from web or iPhone. They are currently working on Android app.
I use PTonthenet.com. It is a bit clunky to use but they have a nice selection of exercises, and you can add your own in as well. You can email or print out the program.
IDEA is reportedly adding their own in the next few months.