Here you go.
- Dratini, from Dragon's Den for getting all questions correct (Gold, Silver, Crystal, HeartGold, SoulSilver)
- Zigzagoon from Pokemon Box event (Ruby, Sapphire)
- Deoxys from Pokemon Ranger and various other events (Gen 4)
- Raikou, Entei, Suicune in various events (Gen 4)
- Pikachu from various events (Gen 5)
- Dragonite from an event (Gen 5)
- Genesect from an event (Gen 5)
Bulbapedia lists all the specific details of the events, I see no point in writing them all out here when you can follow a link and find them that way.