Boundary to Quad

This feature automatically detects the edges of selected UV islands or, if no selection is made, automatically processes all UV islands within the UV tile and places the UV points of these edges on the outer edges of the UV tile.
The Boundary to Circle command is a similar tool that places these points on a circular arc instead.

The outer points of each UV island (left) are moved to positions on the border of the UV tile (shown on the right).