This one may be frightening, but don't freak out. We haven't lost anybody to this workout. Yet!
Death. By. Burpees.
This is an Every Minute On The Minute workout. Start with 5 Burpees the first minute. Rest the remainder of the minute. Then do 6 Burpees the next minute. Rest with whatever time is left. Then 7, 8, 9.... Keep going until you can't finish the Burpees in the minute. When that happens, rest the next full minute, then try to complete the reps you weren't able to finish. If you complete them, move on. If not, rest another minute and try again. How far can you get in 20 minutes??
Don't like burpees? Try this one with Squats, or Sprints, or Pushups. Pick any full body exercise you like and give it a shot.