Flatten Brush
From ZBrush Info
The Flatten brush allows you to easily 'press down' parts of your model into planar surfaces. In addition, you can raise or lower the surface as you flatten it.
Using the Flatten brush, you can add rough flattening to you model, such as enhancing the plane of a model's cheekbones. You can also achieve completely flat surfaces, for mechanical models, walls, etc.
Flatten flattens the surface towards a plane whose angle is determined by the area immediately under the center of the brush. As a result, using this brush with strokes or alphas that change the angles of areas of the surface may cause the flattening plane to shift around. To flatten to a particular plane, ensure you brush over a relatively smooth surface; for example, use the Smooth brush first, if needed.
