The NY Times has an article from this week about training with kettlebells, and how successfully training with them can reduce back pain by training the posterior core muscles that are often weakened by long hours of leaning of desks and computers at work.
The best exercise for relieving back pain is the Kettlebell Swing:
When the Swing is executed correctly, it incorporates every posterior chain muscle, loosens tight hip muscles, and improves abdominal strength. The back is kept straight through the movement and the head is always 'looking down field'.
Knowing the correct weight and proper form are important to correct execution of the Kettlebell Swing requires some flexibility and patience with getting the form.