crop for Mac

From cf/x:

This is a simple, yet effective iMovie plug-in to cut your clip to size. Since a plug-in can't alter the size of a clip (a limitation from iMovie), those parts of the clip that you cut off are set to a color of your choosing.

What should i use it for?

Use this clip to remove unwanted parts of a clip, or to produce areas free of footage where you can place a second (or third) source (e.g. with multiple movie, or chormakey static/movie).