Dobutsu no Mori+
December 14, 2001
In Dobutsu no Mori+ and its revisions (Animal Crossing and Dobutsu no Mori e+), the complete Famicom/NES game Punch Out!! Featuring Mr. Dream is featured as a playable game, but it's normally unaccessable and was made available only through Nintendo's giveaways.
See here for other NES games included in the game.
Animal Crossing: Let's Go to the City (European version) DLC
May 22, 2009
To celebrate the release of the Wii game Punch-Out!! a special gift letter was made available in the European version of Animal Crossing: City Folk between may 22 and June 5, 2009.
Attached to the letter is a green training headgear (called "Green Headgear" in-game) for the player's character to wear.
Copyrights for both series are held by Nintendo.