Tabatas. The longest 20 seconds and the shortest 10 seconds of your life.
Don't lie to me, you have 4 minutes in your day where you can fit in a workout. The other day I had 10 minutes to get a workout in, so I did 2 rounds of Tabatas.
What are Tabatas? It is an exercise protocol where you go as hard as you can for 20 seconds, rest for 10 seconds, and you repeat that sequence for 4 minutes total.You have to use an exercise that is hard.
No single joint exercises like bicep curls or triceps kickbacks. It has to be something that wears you out you like sprinting, burpees, watching political ads, etc... I chose pushups and jump squats for my workout. 10 minutes (4 minutes of pushups, rest 90 seconds, then 4 minutes of jump squats) total and I got a great workout in. Here is a video where I demonstrate Tabata pushups.
The GymBoss (http://www.gymboss.com/ or search for the app on your fancy smart phone) that I mention is really helpful for Tabatas and any kind of circuit training you do. Keeps you on track and keeps you from getting distracted looking at your watch wondering when you can stop the madness.