well it does have a iphone/touch app, but it is also on a web platform. I think Fleetly works great!
it's got preloaded exercises and workout routines (plus you can enter your own exercises and make your own workouts). It can also generate workouts based on the amount of time you have, what equipment you have, what part of the body/type of workout you want to do. On top of that there is a bit of a social element with challenges, comments, friends etc. pretty fun, and has definitely kept me motivated.