This tool replaces a particular kind of node with a rectangular volume of a target node. Ie, you click on the ground and a 3x3x3 mound of dirt appears, which can be other dimensions, materials, etc. Limited to only placing a maximum of 1024 blocks, but this can be changed.

The first three inventory slots determine how this tool behaves. Slot one, two and three determine X, Y and Z dimensions, respectively. Shift + Left click sets which nodes to replace, and Shift + Right click sets which nodes to replace it with. You can select air by clicking anywhere there isn't a block. Left-clicking places nodes in the ground, right clicking places nodes on top of the ground.