WiiConnect24 (partially found video game demos; 2006-2013)
WiiConnect24 was originally a service that offered online experiences to Nintendo Wii users. The service originally launched on November 19th, 2006 and ended on June 28th, 2013.  Some of the services offered included the Everybody Votes channel, Check Mii Out, the Forecast channel, data exchanging with friends, the Nintendo channel, the Mario Kart Wii channel, and most importantly: various downloadable content and demos for the Nintendo DS.
Nintendo announced the WiiConnect24 to the public in mid-2006.  The service enabled the ability to remain connected while the console was on standby mode alongside with being bundled up with numerous services. This included the aforementioned channels and the Wii Message Board. The Wii Message Board allowed users to send messages to each other, Miis, emails, console and software updates, and various game content such as Nintendo DS demos or downloadable content for other specific games. 
However, many of these demos have been lost due to the DS' unique way of downloading and storing them. After a set amount of time or when the system is turned off, the DS deletes the demos. This is due to the Nintendo DS temporarily storing the demos onto its memory. Which means that some games cannot be completed on both the Wii and the Nintendo DS because they require data from demos or other WiiConnect24 services. For an example, the Metroid Prime Trilogy for the Wii will no longer have access to the ability of shared Green Credits (which stops you from unlocking all bonuses). Another example is on Kingdom Hearts Re: Coded for the DS, the game requires WiiConnect24 data to complete the Avatar Gallery and 100% complete the game.
On June 28th, 2013, the WiiConnect24 was discontinued alongside with its various services being unusable including the ability to download demos and other various in-game content. Life support for the Nintendo Wii Shop was extended until January 30th, 2019. 
Current Archival & Revival Efforts
While some demos haven't been recovered yet, many of them have been dumped online and was originally compiled onto a Google Drive folder. The original Google Drive folder, entitled "DS Demos2", has since been removed for unspecified reasons.  Fortunately, a website called davr.org has taken its place and made an extensive archive of the demos. 
After WiiConnect24 was discontinued, the current effort to recover its services have been spearheaded by a project entitled RiiConnect24. RiiConnect24 has begun making various services available once again. These services include the Wii Message Board, the Forecast, the News, and the Everybody Votes channel. Alongside with downloadable content for Animal Crossing: City Folk. Sadly, some services (such as the Wii Shop channel) cannot be continued due to various problems regarding how the service works.
Some games like Mario Kart Wii have another service that allows online connection to live server. This service is known as Wiimmfi. Wiimmfi hosts servers for both Mario Kart Wii and several other games.
- "Discontinuation of WiiConnect24 Services". Nintendo Support. Retrieved 19 Sept '19.
- "Check Mii Out". YouTube. Retrieved 19 Sept '19.
- "E3 2006: Wii Wi-Fi Just Like DS". IGN. Retrieved 19 Sept '19.
- "Wii Shop Channel Discontinuation". Nintendo Support. Retrieved 19 Sept '19.
- "Removed Google Drive Folder". Google Drive. Retrieved 20 Dec '16.
- "Official DS Downloadable WMB Demos". davr.org. Retrieved 19 Sept '19.
- "RiiConnect24 Homepage". RiiConnect24. Retrieved 20 Dec '16.
- "Wiimmfi Main Page". Wiimmfi. Retrieved 19 Sept '19.