Razor Tool
 

This tool slices (splits) the current layer into two or more pieces. The slicing is done by first marking the layer using left-clicks (horizontal) or shift+click (vertical). After all marking has been done, the layer is split by right-clicking (or hitting the Enter key).

This tool can be used for designing tiles for use in resizable web pages or user interfaces, such as the ones shown below. It may also be used for cropping images, although the preferred way of doing so is the dedicated crop tool.