It's the end of an era: Nintendo is bringing its Nintendo Wi-Fi Connection service to a close on May 20, meaning the end of online functionality for over 60 Wii and DS titles.
It's not a complete shutdown of online services for these systems, however -- the Wii's Shop Channel, the DSi Shop and video-on-demand services are also not affected -- however, it does mean that online play, matchmaking and leaderboards will no longer be accessible in the affected titles, and the Nintendo Video service for 3DS is also set to come to an end on March 31.
Nintendo has a full list of the affected software here. Standout titles include Animal Crossing: Wild World (DS) and City Folk (Wii), which offered similar online play and social features to the more recent 3DS title New Leaf; Dragon Quest IX for DS, which had additional content available online; Mario Kart DS and Wii, both of which had online play and leaderboards; the Professor Layton series, which had downloadable extra puzzles; numerous Pokémon games, which had various online features; and Super Smash Brothers Brawl, which had online play and level-sharing.
In many cases, the affected titles have more recent successors on Wii U or 3DS that offer online functionality, though in some cases -- Mario Kart and Smash Brothers being the most prominent -- these sequels are yet to be released.
Nintendo stresses that offline and local play for these games is unaffected, since the online functionality was at no point used as DRM or a requirement for gameplay. It simply means that you will no longer be able to access Internet-based multiplayer or social options from these games. The only item on the list that will cease to function altogether when the service closes on May 20 of this year is the Wii's chat software Wii Speak. The Wii's Internet and YouTube channels will continue to function, however, since these use the open Internet rather than Nintendo's proprietary online services.