The tool lets you assign simple geometric shapes to transforms, then position them at any point in the Maya viewport.
The objects can then be selected by clicking or dragging over them (like a custom heads-up display). For example, you can create facial pickers for character rigs or use it for hard-surface rigging.
Screenspace is free (Apache 2.0 licence) for Maya on Windows, macOS, and Linux. You will have to build the plugin directly from the source code (you can find a guide on GitHub).