The Wii Remote API. “allows the Web page to detect all Wii Remotes that are connected to the Wii [...] this makes it possible to make Web pages interact with up to four users at the same time, a concept not normally possible with traditional JavaScript event detection.”
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It looks like Opera have taken their Wii api offline (the page is now access denied)...though I can't imagine why - it makes for the perfect opportunity to create mini-games that are accessed online.
Good old Google cache came to the rescue!