Throwing a knuckleball is a dying art. The only full-time knuckleball thrower is Tim Wakefield of the Boston Red Sox, who is keeping his career going past age 40 by mastering a pitch that doesn't put much stress on the arm.
That said, it's almost impossible to control, and rough on any catcher. And when it's not thrown exactly right, it's easier than batting practice for good hitters.
The secret behind a knuckleball is to throw the pitch with no rotation on the ball whatsoever. The wind resistance on the ball will make it dart all over the place.