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How do you create the future of Web apps? By accident. Caterina Fake and Stewart Butterfield (above) are the husband-and-wife team behind Flickr, the photo-sharing site that launched in February 2004 and quickly got so popular that Yahoo acquired it a year later. Flickr wasn't born as a fully realized idea--in fact, it wasn't even originally for photo sharing. But its building blocks make up a successful Web 2.0 application. Here's how it evolved.