Boundary to Circle

This allows you to place the edge vertices of selected UV islands—or, if nothing is selected, the border points of all UV islands—simultaneously on circular curves that span the entire UV tile.
If you need a rectangular shape instead of a circle, use Boundary to Quad.

The outer points of each UV island (left) are arranged into a circle that touches the outer edges of the UV tile both horizontally and vertically (shown on the right).