F1(hold) - hide HUD. After that press F10 to make the mouse pointer appear and click again, so you don't need to hold it anymore.
F2 (while holding F1) - take a screenshot. Saved as png in the folder %appdata%\.minecraft\screenshots.
F3 (hold) - option to toggle FPS/debugging.
F5 - third person view.
F10 - freeze the game and make the windows cursor appear. Game windows get inactive.
F11 - toggle fullscreen.