I have a Precision Armaments brake on my AR-10, on my AR-15s I tend to go with the Jerry Miculek style brake like the second pic Rastoff posted as they are cheap and very effective, but damn are they loud for shooters around you, I've found a good compromise to be the Strike Industries J-Comp which is what I use on my primary 223 rifle now.

These TTAG write-ups are a pretty comprehensive roundup of the options:

5.56 Muzzle Brake / Comp / Flash Hider Shootout!

AR-15 Muzzle Brake Shootout #2 -- TheTruthAboutGuns

AR-15 Muzzle Brake Shootout #3 - The Truth About Guns

.30 Caliber Muzzle Brake Shootout - The Truth About Guns