Economy BMX Bicycle Single Speed 16T+ Freewheel Tool (4 prong)
Economy BMX (single speed) Freewheel Removal Tool
4 prong mating, which is the most common freewheel interface (but always check yours first... there are several different types depending on brand and age of the freewheel). Typically if your freewheel is single speed and 16 teeth or larger, this is the tool it uses.
The hex portion can be used with a wrench or with a vice.