That cracking your knuckles causes arthritis is a very common myth unsupported by evidence.
The reason that joints "crack" is that when you pull on them, or put the joint into a certain position, the pressure drops within the synovial fluid that lubricates the joint. This pressure drop encourages a gas bubble (containing mostly carbon dioxide) to form within the joint, taking up about 15% of the joint space. This pushes on the ligaments and other structures that surround the joint making them pop outwards, producing the first of the two noises that you hear when you crack your knuckles.
Next, when the joint is moved again the pressure increases, causing the CO2 bubble to abruptly implode, which produces shockwaves and also sucks back in the ligaments. This we hear as the second cracking / snapping noise. Measurements suggest that the enegry unleashed when this happens is only about 7% of the amount you would need to damage cartilage, making knuckle-cracking an unlikely cause of arthritis.
In terms of clinical studies, one dedicated researcher - Daniel Unger - published a personal report in 1998 (in Arthritis and Rheumatism) of the effect of 50 years of knuckle cracking of his left hand, but not his right hand. He had no excess of arthritis in his left hand compared with his right.
In a slightly larger (and therefore possibly more reliable) study, Jorge Castellanos published a paper in 1990 documenting findings from 350 people who had cracked their joints for 35 years. There was no excess of arthritis in these individuals compared with non-cracking controls!
Arthritis aside, it still may not be a good idea to crack joints because although the 350 people in the study didn't appear to have any joint damage their grip strength was 1/4 of what it should have been.