AtlasViewer allows you to inspect the texture atlases that are currently being used and view details about the sprites on each atlas.
Among other things, this allows you to find out how much space each sprite takes on the atlas and which mods may be wasting space on the atlas with inefficient sprite layouts.

By default, the atlas view is opened by pressing V. The view of the atlas can be panned by dragging with the left mouse button and zoomed with the scroll wheel.
To view the details of a sprite, hover over it in the atlas view and right click it.

Both the full atlas as well as individual sprites can be exported to a PNG file.