Rollpie Perspective is a free drawing tool that runs in your browser with no install. Pick one-, two-, or three-point perspective with the buttons at the top, 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 below 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 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.