The front loaded kettle bell walking lunge. It's more than a mouthful, it's also an extremely effective and simple way to work several important areas of the body simultaneously.

Lots of people get the right ideas when attempting lunges while carrying kettle bells or free weights, but where the idea succeeds, many fail in the technique department and then not only is the effectiveness of the exercise reduced but the potential for injury increases. Without the upper back stabilization when carrying a load at your sides, the shoulders will internally rotate (roll forward) causing tension to be lost and dominoing to a rounded back. If you hold two kettlebells in the front position (or one kettlebell), you can almost assure that you will maintain the proper technique through the upper body simply to keep the weights aloft. I prefer this even to the overhead version as that version allows for more range of motion (movement) through the shoulder joint...and often that means you have a better chance of putting yourself in an incorrect position.

When done properly this exercise:

Increases hip flexibility and stability. Two vital aspects for athletes and non-athletes alike.
Single leg functionality.
Strengthening through the legs and butt. The quads and glutes get the most work but the hamstrings are activated as well. Trust me...you'll realize what was worked the day after.
Core stabilization. By front loading with a kettlebell, you are forced to maintain an upright torso and stabilize throughout the core (abs, obliques, back) as well as upper back stabilization to maintain correct posture throughout the exercise.
Will skyrocket your pulse. The sheer amount of muscles working will force the heart to beat a bit faster to make sure enough oxygen carrying blood gets to where it needs to go.
Did I mention it works the glutes. You know...the ass?

If you don't have access to kettlebells (really?) this can be done holding a free weight in the front position as well but be careful to maintain proper shoulder stability. Think down and back, while activating your lats (try to squeeze the back of your armpit without drawing your elbow in towards the body). This isn't a beginner exercise, so make sure you're comfortable with regular walking lunges before attempting. If you're an old hack at exercises like this? Try it heavy...real heavy. Just make sure the technique is correct!