Based on the Angel Block from the ExtraUtilities mod for Minecraft, the floating anchor, when used (right click like you're placing a block), places a block directly at your feet in mid-air. When broken, it returns to your inventory.
Code licensed under Mozilla Public License 2.0 (MPL-2.0), and the textures under Creative Commons Attribution Share-Alike 4.0 (CC-BY-SA 4.0).