It is the perspective crop tool
To get to it, hold on the the Crop tool to access the fly out menu and then select the "Perspective Crop tool
To use the tool, you have to select 4 corners or the area that you want to crop. But you do not need to draw a perfect right angled rectangle like what the traditional crop tool need you to do.
I find it very useful when we have taken a picture of a rectangular area with your camera phone. Usually the perspective will be wrong in such cases.
Examples would be like
- Signboard on the wall
- A piece of document
- The projected image from a presentation