MoveIt is a free Chrome extension that tries to help you avoid sitting in front of your computer for too long. At intervals of your choosing MoveIt will prompt you to get up and complete a short exercise. You can set the intervals to be as frequent as every five minutes or as infrequent as every hour. You can also disable MoveIt altogether for the times when you absolutely cannot be interrupted.
Sworkit Kids which I featured yesterday provides you with short exercise your students can do in your classroom or at home. Sworkit Kids simply features short video demonstrations of a movement like diagonal hopping accompanied by a countdown timer. There is also a Sworkit app for adults which provides full workouts to follow along with on your phone, tablet, or computer.
GoNoodle is a popular service that has been around for five or six years. It's changed a little bit over the years but at its core it is still designed to promote physical fitness in a fun environment. GoNoodle features lots of free videos that lead students in short, 2-5 minutes, exercises. These are fun exercises like dancing that can be done in your classroom or at home with parents. Many of the videos are also available on GoNoodle's YouTube channel.