Donkey kicks are a great way to gain suppleness as well as lower back and glute tone and strength.
What's involved: Lower back (extensor muscles), glutes, hamstring, triceps, quads.
Powered by: Glutes (primarily), but the hamstrings do come into play as do the lower back muscles on either side of the spine.
Tip: Keep your leg at a perfect 90 degree angle forcing the hamstrings to work to maintain the pose. This way you also get a great hamstring workout.