One of our instructor Brandon Wright's favorite accessories on his M4 was the Magpul Industries Angled Fore Grip.
Our colleague Greg Grant did a post on this back in January after he talked with the rad guys at Magpul during the SHOT Show. But I noticed it on Wright's weapon and decided it was worth getting his opinion on it.
He taught us a method for holding the rifle that forced our thumb almost over the forward grip of the M4. Using that technique really helped with accuracy and that technique, combined with the AFG made the rifle a lot more steady and accurate.
Wright acknowledges the AFG might inhibit use of optics and sensors on the end of the M4 and might not be the best accessory for straight up CQB door kicking, but for shooting on the move and shots with some distance, the AFG gets his vote.