A list of titles that will be losing DLC, or other content, when Nintendo Wi-Fi Connection goes offline.
http://www.nintendoworldreport.com/feature/37033/nintendo-wi-fi-connection-dlc-doomsday-to-do-list
By now, I'm sure you all know that the Nintendo Wi-Fi Connection service will permanently be going offline on May 20, 2014. This means that many Wii and Nintendo DS titles will lose their ability to have functioning online multiplayer modes. Even worse, games that offered unpaid DLC, or had content blocked unless earned through some online means, will never be able to gain access to it again.
In order to help you, and the world, preserve as much as possible before the deadline, we have come up with this handy list of Wii and DS titles that will be impacted by the loss of DLC, or other content. We hope that you are able to connect online to get this content before it is lost.
Nintendo Wi-Fi Connection will forever live on in our hearts, and hopefully the downloaded content will live on in some form. It would be a shame if the DLC wasn't preserved for historical purposes, so definitely help out by doing what you can before it's too late.
(Note: If you see a title missing from this list, feel free to contact us so we can add it.)

Wii
Animal Crossing: City Folk
Get all the DLC from friends and other players online before it's too late.
(Note: It will still be possible to get the DLC after the Nintendo Wi-Fi Connection service ends, but you will have to do it by traveling to other people's towns, who have the DLC, through the Nintendo DS Suitcase feature. This will be difficult for most players.)
Excitebike: World Rally
Certain bike colors are only available to those who earn points through online races.
The Last Story
Several items and gear and only available to those who play the online multiplayer mode.
WarioWare D.I.Y. Showcase
Get downloadable games from NinSoft. Includes some special levels made by big name game designers.
(Note: There isn't enough space to save all the DLC to this game. It is, however, possible to split the DLC between this and WarioWare D.I.Y. on Nintendo DS.)

Nintendo DS
100 Classic Books
Download extra books.
Dragon Quest IX: Sentinels of the Starry Skies
Download special quests and guests.
Final Fantasy III
Send letters to friends over Mognet in order to unlock the Onion Knight job class, Legendary Blacksmith items, and a secret dungeon.
(Note: It may be possible to still unlock these at a later date though direct DS to DS communication with the title)
Fire Emblem: Shadow Dragon
Last chance to visit the online shop for exclusive items such as the Brave Weapons and the Elysian Whip (the only way to access the Falcon Knight class).
Fossil Fighters: Champions
Download exclusive fossils.
Grand Theft Auto: Chinatown Wars
Unlock special Missions.
Jam with the Band
Download extra songs.
(Note: If you have further information about how many songs there are, and how many game carts are needed, let us know.)
Mario vs. Donkey Kong 2: March of the Minis
Download levels.
Mario vs. Donkey Kong: Mini-Land Mayhem!
Download levels.
Mario vs. Donkey Kong: Minis March Again!
Download levels.
Picross DS
Download Puzzles.
(Note: It will take 10 copies of the game to download all the official content.)
Picross 3D
Download Puzzles.
(Note: It will take two copies of the game to be able to store all the DLC.)
Pokémon Conquest
Download Episodes.
Pokémon Ranger: Guardian Signs
Download Missions.
Pokémon Ranger: Shadows of Almia
Download Missions.
Professor Layton and the Curious Village
Download all the Weekly Puzzles.
Professor Layton and the Diabolical Box
Download all the Weekly Puzzles.
Professor Layton and the Last Specter
Download all the Weekly Puzzles.
Professor Layton and the Unwound Future
Download all the Weekly Puzzles.
In London Life, send/receive enough gifts from friends in Segal's Social Club to unlock rare items and special visitors.
Style Savvy
Download special clothing.
WarioWare D.I.Y.
Get downloadable games from NinSoft. Includes some special levels made by big name game designers.
(Note: It will take an estimated two copies of the title to hold all the DLC games. It is also possible split the DLC between this and WarioWare D.I.Y. Showcase.)