Like Pokémon Platinum, there are a variety of in-game events activated when you send over a certain Pokémon you obtain from events. These tend to allow you to get access to new Pokémon
The first one of note would be the Arceus obtained from the events. If you send this over to Heart Gold & Soul Silver, when you reach the Ruins of Alph with the Arceus, a new area will open where you will meet Cynthia from Sinnoh. You have the ability here to select one of three Pokémon; Dialga, Palkia or Giratina, to have at Level 1. Click here for further details.
|Pikachu Coloured Pichu
The second one of note would be the Pikachu Coloured Pichu obtained from the events. If you send this over to Heart Gold & Soul Silver, when you reach Ilex Forest with the Pikachu Coloured Pichu in the front of your party, you will encounter the Spiky Eared Pichu at the Ilex Shrine. After a small bit of play with your Pikachu Coloured Pichu, the Spiky Eared Pichu will join your party. Click here for further details.
Next is the Jirachi that you obtained from an event. If you send this Jirachi over to HeartGold & SoulSilver, you will be able to unlock the special Night Sky's Edge Pokéwalker course. This course has a variety of Pokémon and items for you to obtain on the Pokéwalker. Click here for further details
Finally is the Celebi to be given away in 2010. If you send this Celebi over to HeartGold/SoulSilver, or get sent it directly, take it to Ilex Forest's shrine and it will begin a side-story to give you background information on a variety of aspects including your Rival and Team Rocket. Click here for further details