The kettlebell (a cannonball with a handle) was first used as a Russian training device over 300 years ago, so though it might seem like just another gimmick in a long line of fitness fads, it's the real deal.  Along with the hunk of iron that we bought Jake for his birthday, we gave him a copy of Enter the Kettlebell.  Its author, Pavel, a former special forces trainer in Russia, is credited with popularizing the kettlebell in the USA.  Chief among its virtues, at least in my mind, is that the kettlebell develops strength while simultaneously powering up the heart.  Or, as Pavel puts it so wittily, you develop your heart "without the dishonor of cardio."