Reducing agents:

carbon (C)

carbon monoxide (CO)

hydrogen (H₂)

hydrogen sulfide (H₂S)

metals

potassium iodide (KI)

sulfur dioxide (SO₂)

ammonia (NH₃)

hydrogen peroxide (H₂O₂)
(can act as oxidizing agent as well)

Oxidizing agents:

fluorine (F₂)

chlorine (Cl₂)

bromine (Br₂)

iodine (I₂)

concentrated sulfuric acid (H₂SO₄)

nitric acid (HNO₃)

oxygen (O₂)

potassium manganate(VII) (KMnO₄)

potassium dichromate(VI) (K₂Cr₂O₇)

hydrogen peroxide (H₂O₂)
(can act as reducing agent as well. It is stronger as an oxidizing agent, especially in acidic solutions)