2 items: Exp. Share and Lucky Egg.
Exp. Share shares the Experience out equally between Pokemon, even if they do not take part in the battle themselves.
Read this for more.
DPPt Route 206 (Rowan's assistant), Jubilife TV Pokémon Lottery Corner (3 digits)
HGSS Mr. Pokémon (exchange for the Red Scale), Goldenrod Radio Tower Pokémon Lottery Corner (3 digits)
BW Castelia City (Battle Company), gift from the Pokémon Fan Club Chairman
B2W2 Castelia City (Battle Company), gift from the Pokémon Fan Club Chairman, Floccesy Town (held by the Pokémon received from Benga after defeating him)
Lucky Egg Increases the experience gained by the holder to the equivalent of boosted experience. This only works for the Pokemon that battled.
DPPt Held by wild Chansey
HGSS Held by wild Chansey
BW Chargestone Cave, Pickup, Route 13, held by wild Happiny
B2W2 Celestial Tower
Well, hope I helped! :)