Place vertices to draw a polygon, then pick a side to annotate it with a Bézier leader line that shows its length. It is handy for the kind of figures seen in elementary math and entrance exams.
Drag a vertex with the mouse to move it. Dragging empty space pans the whole figure, and the wheel zooms around the cursor. The panel names in the top right open and close each panel, and you can grab a panel header to move it anywhere. Double-click a panel title to switch to the fixed layout, which shows a single panel in the top right.
Fine-tune the leader line's color, width, and offset, and the label's size, position, and angle from the panels. Relative and absolute scaling are supported, so you can finish clean, textbook-style diagrams.