Draw Math Figures is a free drawing tool that runs in your browser with no install. 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.
Fine-tune the leader line's color, width, and offset, and the label's size, position, and angle from the tabs. Relative and absolute scaling are supported, so you can finish clean, textbook-style diagrams.