Pull ups, chin ups, and the raw power that they build.
Pull ups (and chin ups) are an exercise that every serious strength athlete needs to master. They build brute, raw, unadulterated STRENGTH in the upper body -the kind that you cannot get from lifting weights, and certainly not from working out on machines in the gym.
This exercise is more than worth it's weight in gold, but is unfortunately ignored these days by most people. Let's face it, when was the last time you saw someone working his butt off on a tough set of pull-ups? Personally, the only thing I've seem people do on the monkey bars in the local parks around where I live (and by the way, those monkey bars are great for doing pull-ups) is hang their jackets on them, or loiter by them while yammering on about the latest exercise fad. And that pretty much explains why most people have weak backs, and suffer from a plethora of back related problems.
Pull ups build the back muscles to the fullest. The back is the true foundation of super strength in the upper body (as opposed to the chest, which most people are obsessed about), and this one exercise will work it to the max. Pull-ups also build a powerful grip, and strengthen the forearms beyond belief. And the benefits do not stop there - the biceps, triceps, chest, and core are all heavily taxed during the movement. If there was only one upper body movement that you could do, that would give you the biggest "bang for your buck", I would advocate the pull up (or chin-up).
Pull ups also provide a true test of one's strength. How many people these days can complete ten, proper pull-ups in one set, regardless of their bodyweight? Not very many I'd bet, and this includes the "muscleheads" at the local gym as well. There is a very good reason the pull-up is a mandatory requirement for the US Marines and other elite forces around the world - that being that the exercise builds true strength, and lasting endurance.
Remember that pull ups should always be done in proper form to gain the utmost out of every repetition. This form is detailed in Fast and Furious Fitness; be sure and learn correct form before attempting to progress in this exercise. You should never use momentum to complete a rep. Allow your MUSCLES to complete the movement, rather than your ego.
Most people avoid pull ups because they find them to be too tough, and are forever stuck on the lat pull down machines while achieving little by the way of back development and true strength. Don't be one of these people. Master the pull-up and it's variations fully, and you will move to an elite level of strength that few people these days will ever attain in a lifetime of training.