You may know that squatting is the king of all weight lifting exercises. (Deadlifting is #2.) No other exercise leads to as much muscle development and overall gain than squats, especially when you consider most of your muscles are in your legs. Most people you see squatting in the gym are actually using poor form because they are not squatting below parallel.

As you can see, this guy's butt is going lower than his knees. That is a proper squat. Doing less than that is called a partial squat, and is actually worse for your knees (because of which muscles are activated) and back (because of your spine's orientation).
Some people are unable to do a proper ATG (ass to ground) squat because of flexibility issues or injuries. A great alternative to the classic ATG squat is the box squat.
I did my first box squat this week and HOLY SHIT, what a workout. I have maxed out the leg press (315 lb) and leg curl (250 lb) machines at my gym, so I figured that there was not a ton I could do to improve my legs at the gym. However, the box squat was probably the most strenuous exercise I've done other than straight leg deadlifts.
The technique itself is simple. Bring a bench over to the squat rack and place it behind you. When you squat, keep your feet and knees forward and (slowly and steadily!) sit down on the bench. Make sure you are actually sitting on it and not just tapping your ass on the bench, because this redistribution of weight is key to the exercise. After 1-2 seconds of sitting, EXPLODE up as hard as you can, keeping knees and feet forward. Since you actually went to static equilibrium when you sat on the bench, getting back onto your feet builds muscle that you won't stress as much doing ATG squats. It's a great way to get a better jump or improve your other compound movements (bench, deadlift, press, etc). Plus there is no "guesswork" as to whether you squatted far enough or not.
In summation, box squats are crazy hard but fun and a great exercise. Give them a try.