I'll play: A CO2 regulator for dispensing beer (as opposed to one used for welding, for example).
Most folks use CO2 tanks with CGA-320 valves, so you'd want to get a primary regulator like one of these.
If you're thinking of using a paint ball tank, there are adapters that screw onto that style tank that then let you couple a conventional CGA-320 style regulator thereto, like this...