Mob masher is a machine used to kill mobs automatically. The masher does not require power to run, only a redstone signal which can be activated with a vanilla lever. The masher does not contain an internal inventory so to collect drops an absorption hopper, vacuum chest, or similar should be used.
The masher can be upgraded to behead mobs (where possible), set them on fire, obtain more drops, or 'smite' them for more efficiency. Right-click on the masher to access the GUI.
Beware that the masher does not distinguish between players and hostile mobs so turn off the machine before transversing nearby.