Drag the points of a polygon, circle or inset shape over an image and copy the clip-path value.

A visual clip-path maker by Bennett Feely. You pick a preset or custom shape, drag its points over a sample image, and copy the clip-path value it writes for you.
It covers polygon, circle, ellipse and inset shapes, including rounded corners on inset(). The page notes that you can animate between two clip-path shapes when they have the same number of points.
It points to Can I Use for current browser support. The source is on GitHub, though no licence is stated.
clip-path