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The Symphony CMS team dropped me a line to let me know that they've just launched a new website, symphonyextensions.com, to aggregate all of the 350+ Symphony extensions that exist on GitHub.
They've developed our own XML schema for documenting extension metadata (changelogs, versions, dependencies, authors etc.), which is similar to the packages.json that the node.js world is inheriting, so that this website, and one day Symphony, can consume and intelligently parse it.
The site allows developers to add their repositories in one click (after signing in with GitHub), and allows site builders to quickly discover new extensions by recency of update or by popularity. Check it out.