Minimap
The small overview map tucked into a corner of the screen, showing the surroundings, the player and whatever is worth knowing about.
There are two main ways to build one. One is to render the world a second time with an orthographic camera looking straight down into a texture, then draw that texture in the HUD — simple, always current, but a whole extra render pass. The other is a pre-drawn map image on which the engine merely places icons for players, enemies and objectives, which is cheaper and easier to keep legible. Defender (1981) already had a "scanner" showing the entire level, and Doom's automap (1993) drew walls as lines only once you had seen them.
The design questions outnumber the technical ones: should it rotate with the player or keep north up, should fog of war cover it, and how much can it reveal before players stop watching the game and just stare at the corner.