Development Seed has consistently produced stunning work, both for clients and in terms of their open-source contributions, but TileMill really feels like their crowning achievement. (And all of it in PHP/Drupal, gasp!) Kudos, guys.
From the article:
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Under the hood of TileMill is a combination of open source projects - Mapnik for map rendering, express as a server-side framework, Backbone.js for the client-side, CodeMirror in the editor, carto for the styling language, node.js, and many others.