Most open source wiki and CMS engines host user-contributed themes/plugins in directories whose code, strikingly, is not itself released as open source software. Possibly the best example is WordPress' own plugin/theme directory, offering support for one-click plugin installation as well as plugin metadata, screenshots, changelogs, system requirements information, community rating, categories/tags and so on. The WordPress plugin directory is built on a collection of open source software and works as an interface to an SVN repository, allowing contributors to self-maintain their code.
Is anyone aware of an open source engine with similar features to allow the hosting and community-driven maintenance of themes and plugins?