When someone tells you that cracking your knuckles will lead to arthritis, they're wrong.
What exactly makes that "popping" sound? When you crack your knuckles, or back, or whatever, you are pushing the joint into or out of its normal position. Joints are the meeting points of 2 separate bones. A thick liquid called synovial fluid is found between the bones. When you stretch, twist, push, or pull those bones apart, the pressure is reduced on that fluid, and bubbles form and quickly burst - making that familiar "pop."
Research from the Johns Hopkins School of Medicine shows that people who crack their back, neck, knuckles or knees generally have healthier joints than those who don't. Well, actually, people who tend to pop their joints are more likely to be more physically active than non-crackers.
So go ahead and crack away. It may be annoying to others, but at least it's not hurting you.