I pop my knuckles fairly frequently, and I have been told on several occasions that doing so is bad for your joints. Is this true? Have there been any medical studies on this?
It seems to me like the concern about the negative impact of knuckle cracking brought up in the literature is if it leads to arthritis. Which of course isn't the only harmful thing it could cause.
The Ig Nobel in medicine was in 2009 awarded to Donald L. Unger for research around this very thing. Unger cracked his left hand knuckle systematically for 60 years while leaving his right hand uncracked. Showing that in at least his case it didn't cause any harm. I believe he published his result here.
Beyond that there seem to be two larger studies. The first one published in the British Medical Journal titled Effect of habitual knuckle cracking on hand function shown that there is indeed some problems caused by cracking knuckles:
The second one published in the western journal of medicine titled The Consequences of Habitual Knuckle Cracking examined 28 people in a Jewish home for the aged, with an x-ray. It finds no harm:
So it seems like the the answer is that there's some harm, but it's probably so negligible it's not worth worrying about.