Secret number 1: Spiky Eared Pichu.continue →
During February, Gamestop gave out the special Pikachu colored Pichu to people via wireless connection. Once traded into HeartGold or SoulSilver, if you go to Ilex forest with it in your party and 'check' the shrine a Spiky Eared Pichu will come out to play with your Shiny one. An old man will appear telling you to take the little guy, and don't be alarmed because if you don't have a free space you can go back to Azelea [sp] and return. will be Lv. 30 with the moveset : Volt Tackle, Pain Share, Helping Hand, and Swagger.
Volt Tackle and Pain share is a very good move combo to use.
Number 2: Jirachi
Again, distributed via Gamestop [or some other area], once traded in opens a new PokeWalker route.
Number 3: Dialga, Palkia, or Giratina.
Must trade in a LEGIT Arceus [ie an event one] first, and have it the only Pokemon in your party. After doing so, try to walk into the research lab in the Ruins of Alph and the director will stop you. A dialogue [sp] will continue through and you will be mysteriously warped to the Shinjoh Ruins. There you will meet Cynthia once again and she will talk to you and you will have the chance to meet either a Palkia, Dialga, or Giratina. Mind you they all are holding their orbs, and this is the only way to obtain the Griseous orb in HeartGold or SoulSilver.
No battling is necessary, they simply join your party at Lv. 1 with the move-set Dragonbreath and Scary Face for all three.
Number 4: Celebi
This one I don't actually know about, because I don't have a legit Celebi, but I know if you have it as your lead Pokemon, and go to see the same shrine as with the Pichu, you will be transported back in time to see a short back-story to your rival and the famous Giovanni. You then gain the honor to battle him, and will be brought back you your own time.
Finally Number 5: Shaymin Sky Forme.
Again, this is one where I didn't get the event Pokemon, so I don't know first-hand, but if you trade in your event Shaymin, and show it to a woman in the flower shop in Goldenrod, she will give your the Gracedea Flower
There is also the story event in which you can catch Groudon (SoulSilver) or Kyogre (HeartGold) after beating all Kanto Gyms. If you can get a friend to lend you their opposite Pokemon, you can go back to that place where you got it [unknown to me as I am only on my 8th Badge in Jhoto], you can catch a Rayquaza. But they must be from HeartGold or SoulSilver.