I have a gym at work and they recently got an outside guy to do kettlebell classes.

Here are a couple of links describing kettlebells. Basically they look like a cannonball with an oversized handle. The purpose of the offset handle is to create instability and it forces you to use many more muscles to hold the kettlebell than if you were holding a dumbell and certainly much better than weight machines. The exercises are also dynamic and involve swinging motions. Whereas traditional lifting is more static in nature.

http://en.wikipedia.org/wiki/Kettlebells

http://www.russiankettlebell.com/?c=kb001

Okay, so this is the day after my first class and I am sore from head to toe. Keep in mind I regularly lift and ride 6 - 10 hours a week. The actual class felt easy.......but clearly it was a good workout since I don't normally get sore after lifting or riding.