I made a little layer of basic visualisations for OpenGL, which I called the imaginative name: OpenGL Debug Draw.
It is inconveniently hard to draw basic visualisation things in modern OpenGL; 3d and 2d lines, points, boxes, etc. The sort of things that would be incredibly powerful for hardware accelerated visualisation, and debugging mathematics and logic over the top of games and more complex things where this can often be guesswork. These features are all deprecated and have very unreliable, poorly performing support in modern GL.
I was flicking through Game Engine Architecture, Second Edition and one chapter suggests having a layer of debug visualisation tools to help designers show vectors, bounding boxes, spheres, points etc. Often 3d maths and things like collisions can rely on some guess work, and it's always a good idea to show what's happening in the model and how it matches to the 3d graphics as we see them. It can also be helpful to show why lighting equations are producing strange results on certain meshes - perhaps the normals are wrong in part of the mesh.
In earlier times I had used the immediate mode line drawing facilities built into graphics engines, which at some point come unstuck as they are typically not batched together to fit into a single drawing call - each line or box is drawn in a separate drawing operation and it quickly bogs down and interferes with the pace of whatever real-time interaction you want to observe. So my main design feature was to stuff all of these lines into one buffer so they are all drawn together. New lines can be added onto the stack of lines drawn, and all lines added after an arbitrary point can be discarded, but there's no real dynamic adding and removing, which means no proper frustum culling can be done. I feel this is fine for most debugging tasks (even quite intensive stuff). I didn't want to use modern uniform buffers and more complex data types so that I could support even very old GL versions, which I use for some things, because the portability of modern GL is abysmal.
I considered an arrow-head design to show which way vectors were pointing, but it was easier to just colour the start point of these lines black, with a gradient to a given colour at the other end.
In my game I already debug radius-of-effect things like ambient sounds with a semi-transparent sphere, but it isn't ideal. A 2d circle with a radius line from the centre shown would work in more views.
I'm using GL_LINES but TRIANGLES would be more efficient on a range of drivers - I would just have to abtract the line-drawing API so that it is rendered with appropriately-sized triangle strips. Alternatively, I could write a new 2d rendering API that draws on to a framebuffer texture and renders in an additional pass (this would be pretty neat actually). It would be much easier to compute arrow-heads, nice vector lines, and circles in a 2d API.
Conversely, I could make the library even more minimal, and potentially just a header file with as little code as possible in it to get lines to screen.