I found this on askalice.com concerning cracking knuckles.
This site seems pretty legit. So what's the final word? Someone has got to know more about the subject.
"On the other hand, escaping gas seems pretty innocuous, right? Wrong. Mom wasn't too far off when she told you that cracking your knuckles would make them big. By cracking your knuckles, what you are doing is pushing the joint beyond its physiological stop sign, or further than its range of motion would normally allow for. This puts undue stress on the ligaments and tendons that hold the joint together and in place. Ligaments behave a little like rubber bands---you can pull and stretch them a lot; but, eventually they won't return to their original shape and length.
After years of pushing your joint past its normal range of motion, it may become "hypermobile." If the joint is very mobile (as in the hand), the joint surfaces might wear out. If this happens, the body responds by laying down more bone. Although this sounds like the body healing itself, in reality, matters are made worse. Usually, the bone is not laid down smoothly and the joint surfaces become rough. Worst case scenario - spurring or sharp spots will develop in the bone of the joint in question. Eventually, you may have rough surfaces moving against each other, to which the body might respond by swelling, increasing the amount of fluid and blood in the area. Talk about unpleasant friction! In the end, you could have a joint ("arthro") that is inflamed ("-itis").
Of course, little research has been carried out in this area, so it's difficult to say with a high degree of certainty that knuckle and other joint cracking will lead to arthritis or other joint diseases. But it's fairly safe to say that repeatedly pushing a joint beyond its normal physical range is not a good thing to do. And years down the road, you could possibly end up with big problems, not just big knuckles like mom said!"