BodBot - Your own personal trainer, nutritionist and coach. Free.

Those interested should also take a look at Rise (http://www.rise.us, http://www.producthunt.co/posts/543). It launched recently, offering 1:1 fitness and nutrition guidance from a personal trainer. I know @Noah_L is a fan. :)

I think there's a lot of room to improve on the design side but I've been very impressed with how their fitness plans actually adapt to your needs & progress. System actually reacts to the same stuff you'd discuss with a human coach - what's your goal, how much time do you have to work out, how quickly are you making progress, etc. Even lets you specify injuries so it can avoid certain exercises.