It's not quite what you suggest, but http://www.konkura.com is run by my brother - it lets people create fitness-related challenges (which can be entire workouts or even workout schedules, or just a single exercise) to share with friends. You can compete with people who have similar interests around the world, or keep challenges private to just a circle of friends who have a common fitness goal. There's a wide range of challenges already created by fitness enthusiasts so there's always something new to do. Worth checking out.
