Rollpie Perspective is a free drawing tool that runs in your browser with no install. Pick one-, two-, or three-point perspective in the Display panel, then drag the red vanishing points; where the lines gather is what sets the sense of depth. The handle on the right raises and lowers the horizon (eye level), and the switches in the Display panel toggle the perspective floor grid and the base grid.
Drag a box to move it, and click it to select it and adjust its width and height with the sliders. Add more boxes with the button in the Boxes panel and they share the same vanishing points, so their sizes stay consistent and you can see how depth ties them together. Guide lines run from the selected box toward the vanishing points, so the direction of convergence is clear at a glance.
The panel names at the top right open and close the Display, Boxes, and How to Use panels. Double-click a panel header title to switch how panels are arranged: in floating mode you can drag any number of panels wherever you like, and in fixed mode only the chosen panel is shown at the top right.