The Kettlebell is a cast iron weight, which resembles a basketball with a handle.  A Russian exercise device used for more than 100 years, Kettlebells have long been a favorite in that country for those seeking a special edge in strength and endurance.

In the twentieth century Soviet science discovered that repetition Kettlebell lifting is one of the best tools for all around physical development. (Voropayev, 1983) observed two groups of college students over a period of a few years. A standard battery of the armed forces PT tests was used: pullups, a standing broad jump, a 100m sprint, and a 1k run. The control group followed the typical university physical training program which was military oriented and emphasized the above exercises. The experimental group just lifted Kettlebells. In spite of the lack of practice on the tested drills, the Kettlebell group showed better scores in every one of them.

There was more. Surprised researchers at the famous Lesgaft Physical Culture Institute in Leningrad (Vinogradov & Lukyanov, 1986) found a very high correlation between the Kettlebell lift total and a great range of dissimilar tests: strength, measured with the three powerlifts and grip strength; strength endurance, measured with pullups and parallel bar dips; general endurance, determined by a 1000 meter run; work capacity and balance, measured with special tests!

In addition to their many mentioned benefits, the official Kettlebell lifts also develop the ability to absorb ballistic shocks. If you want to develop your ability to take impact try the official K-bell lifts. The repetitive ballistic shock builds extremely strong tendons and ligaments.

The ballistic blasts of kettlebell exercise become an excellent conditioning tool for athletes from rough sports like kickboxing, wrestling, and football. The extreme metabolic cost of high rep Kettlebell workouts will put your unwanted fat on a fire sale.

Reference:  Dragon Door - Strength Training with Kettlebells

Do-it-yourself Kettlebell design

This design can be used as an adjustable kettlebell, allowing weights up to 35 lbs. 

Parts:

1. 1-1"x 6" steel nipple (handle, external diameter is 1 and 1/4")
2. 2-1" 90 degree elbows (connect handle to vertical pieces)
3. 2-1"x 4 1/2" steel nipple (vertical pieces), longer vertical pieces will allow the use of 
       larger, heavier plates
4. 2-1" 'T's
5. 1-1"x12' steel nipple (through plates)
6. 2-1' end caps
7. 6-5 pound standard barbell plates (make sure the 3/4" pipe goes through all of these, the hole in some of my 5 pound plates are too small to use)

Option 2:  "T-bar" Kettlebell design

Here's another cheap and easy kettlebell plan that I recently made.  This one is especially good for those two-arm swings.  3 pieces of 3/4" pipe, a T fitting, a flange, and a couple of end caps and you're ready to pile on some of those standard plates that you've had laying around.  This is an excellent conditioning tool to add to your arsenal! 

As with some dumbbells, always verify that your collars (or end caps) are tight before using.