Apple - Cursor as full screen crosshairs

While it has many more features than this, something like xScope sounds great for this.

Some main features:

• mirroring
• getting on screen dimensions
• rulers
• guides
• frames
• crosshair

Update: QuickLens has an excellent set of tools: a full screen live crosshair, movable/lockable guides, ruler and a few other measurement, magnification and alignment tools. It was recently (Sep 2020) rewritten as a 2.x release, ready for Catalina and Big Sur. This is my current favorite.

PixelSnap 2 is a well designed app that provides great crosshair & measurement tools. It's a bit costly for just a crosshair, but you can get it now as part of Setapp if you happen to subscribe to that.

Red Lines Tools (App Store, currently free as of 8/2020). It has crosshairs, a magnifier, grid/image overlays, adjustable keybindings, and many other customizations. There are a few bugs but it gets the job done.

Another possible solution: Monosnap is a free screen capture app that displays full-screen crosshairs when capturing an area. You can trigger it via a hotkey and then hit ESC to cancel without actually taking a screenshot if all you want to do is use the crosshairs for alignment.

As others have mentioned, xScope is another option, although it's costly if all you need is crosshairs.

There used to be a small and efficient app called Euclid written by Nicholas Jensen that was specifically for crosshairs. Unfortunately, it's been abandoned.