Nintendo is to discontinue the Nintendo Wi-Fi Connection Service, leaving certain online services for the DS, DSi and Wii consoles unavailable.
A notice on Nintendo's website states: "As of May 20th, 2014, the Nintendo Wi-Fi Connection service will be discontinued and it will no longer be possible to use online features of Nintendo DS/DSi and Wii software such as online play, matchmaking and leaderboards."
A number of services will remain unaffected, including:
Nintendo DS Browser
Nintendo DSi Browser
Nintendo DSi Shop
Nintendo Wi-Fi Connection Pay & Play
Wii Shop Channel
Wii U and 3DS games won't be affected, but online functionality of Wii and DS games played on the systems won't be available.
There's a list of affected first-party games available over on Nintendo's website