My right knee pops quite often, although there is no pain with the pop. I can also pop/crack my fingers and knuckles if I want to, but I don't make a habit of performing that particular action.

I don't have a link to the page, but I remember reading an internet study article a few months ago concerning this subject. The scientists involved with the study performed checks on a wide range of people both young and old, some of which had arthritis, and some of which didn't. According to the results of the survey, clicking/cracking/popping your bones had no proven effect on the human body. Many people who suffered from arthritis did not have bones that clicked and popped. Links to other surveys/studies were also provided, none of which supported the theory that cracking your bones lead to any sort of future illness.

Needless to say, it put my mind at rest.