This move is incredibly simple, doesn't require a ton of equipment, is easily modified, and most importantly lights up your core and triceps. Don't confuse simple for easy though. The L-sit hold is brutal and will reveal any weaknesses you have in your upper body and core.
To do it, place two parallette bars shoulder-width apart and sit in between them. Press your hands down into the parallettes to lift your hips off the ground and extend your legs out in front of you making a 90-degree angle or an L. Hold this position for as long as you can without letting your legs dip down. If you haven't practiced this type of movement, it won't be very long.
Don't have access to parallettes? Try using a pair of plyo boxes or a pair of benches. Any pair of things that can hold your bodyweight should be fine as long as they won't move out from under you.
If the L-sit hold is too tough, bring your knees in toward your chest, and build stability in that position before trying the more advanced version.