QOL Tools: HUD - Distances is a Blender add-on that displays a realtime, at-a-glance measurement between any two selected things. As soon as you have two items selected — objects, lights, vertices, edges and more — the HUD appears on screen with the distance between them.
If you plan to move either item, you can switch the tool to locked mode. While locked, it stays tied to those two selections and automatically updates the measurement as you move or animate them. A distance modes control lets you toggle the helper line style between on, off, or all axes, and read the distance along a single X, Y or Z axis or all at once. The distance modes toggle icon sits at the top-right of the 3D view.
The add-on supports Blender 4.2, 4.1, 4.0, 3.6, 3.5 and 3.4, now including a 4.2 extension version, and is released under the GPL license. The developer mentions using it for architectural and engineering previs. It is also included in the QOL Tools: Full Collection package.