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)