Popularly called Fire Hydrants, this is a great exercise to strengthen your glutes. Position yourself on all fours. Keeping your core tight and knee bent, lift your right leg laterally (to the side). Imagine a dog lifting its leg to mark a tree (or fire hydrant).  Repeat with same leg for the number of reps, then repeat the cycle with your left leg.

Advanced modification: place a dumbbell weight inside your knee and hold it with your leg muscles while lifting your leg