Occasionally a would-be exerciser has little or no access to equipment.
Generally, I point them here:
Simplefit is basically a stripped-down circuit. Nothing but pullups, pushups, and squats. They've split them up into levels, so you can start off easy and work up as you improve. It's three workouts a week - max rounds in a certain time on the first day, minimum time for X rounds on the next workout day, and then minimum time for one round on the third workout day.
It's not going to get your maximum muscle, or take you from weak man to Strongman, but it's an excellent place to start if you've got nothing but your bodyweight to work with. You'll gain strength until pullups and pushups and squats are easy, then you'll build up endurance. But for some people, this sort of workout might be all they need.
It's worth a look, even if just to bookmark "just in case" you need a workout plan for a vacation or unexpected gym absence.