Everyone has cracked a knuckle at least once, even if it was by accident at some point during their lives. It makes a loud popping sound, and often provides temporary relief of sore finger joints.
No one has ever really said that it's good for you, but medical research has reported that cracking away doesn't cause arthritis to develop anymore than those who never do it. Everything in moderation, right? If you feel your knuckle cracking habit is obsessive, speak with a chiropractor or your family doctor to ensure that you aren't causing any excessive harm.