The directories at Backflip, Hotlinks and Octopus are hardly useful at all because of meaningless link titles, weak taxonomies and lack of updates. SuperOpenDirectory has much to do to become a useful resource (provided I'm proved wrong, having my doubts about how many people really want to actively publish a directory, as opposed to using a bookmark manager or metabrowser that happens to build a directory in the background). The fact that it exposes the underlying XML (formated in OPML) is a good first step. A search engine is a must (Google?), since SOD (sic) is based on outlines, it might turn results into structures too (as does Vivisimo, a Carneggie Mellon spinoff which recently released an improved beta).