Nudge Brush
From ZBrush Info
The nudge brush allows you to move vertices around while (to the extent permitted by the density of your model) those vertices remain on the existing surface. Contrast this to the Move brush, which moves vertices in the screen's xy plane, without paying any attention to the underlying surface.
For instance...
Think of adding a knothole to a tree trunk. You'll likely want the original geometry of the trunk to flow around the new knothole, so you could use the Magnify brush to quickly move vertices out from the center of the knothole, do a local subdivision to add more geometry in the knothole area, and then use Nudge to get that geometry tuned just right for the slightly irregular shape of the knothole.
Think of adding a knothole to a tree trunk. You'll likely want the original geometry of the trunk to flow around the new knothole, so you could use the Magnify brush to quickly move vertices out from the center of the knothole, do a local subdivision to add more geometry in the knothole area, and then use Nudge to get that geometry tuned just right for the slightly irregular shape of the knothole.
