Sanke Coupler - D Style.
This is the coupler you want for most of the kegs purchased from a microbrewery or liquor store. This couplers comes with a 5/16th hose barb for your gas line but requires a beer line tailpiece kit. The built in pressure relief valve (50 PSI) prevent over-pressurization and it's heavy duty construction is designed to withstand commercial usage. Beer is dispensed out of the top port, the side port is for gas. Most domestic beers will use this coupler but please confirm the one you need before you buy it.