Push Up with Rotation
What it strengthens: 
Triceps, shoulders, chest, abs, back

Cassandra's Note: 
Because of the twisting and balancing on one arm involved, this push-up really engages your core, increases your coordination, and adds a beneficial stability challenge to your shoulders. 

This is done just like the wide grip push up, but with a twist (literally). Perform your push-up and on the way up, shift your weight to one arm as you lift the other arm out to the side (making your body a T-shape). Pull your shoulder blades together and reach your arm out and back as far as comfortable, then rotate back to start. On your next rep, rotate to the other side. Each push-up is one repetition (don’t count each side). Move with control.

To make easier: 
Do not bend elbows as far and/or use a higher surface or wall.

To make harder: 
Use a lower surface or do them on the floor.