CodeDown is a coding and technical writing notebook based on Markdown. At its core it is a fully style-able WYSIWYG Markdown and LaTeX editor, with collaboration features so you can work on documents together with teammates. Furthermore, every Markdown code block becomes executable, so you can run the code at the push of a button and have its results appear directly in the document. It supports a wide array of coding languages, and can be run both as a desktop app and in an online hosted "sandbox." It can be used for homework, data analysis, writing reports or research papers, or general technical work.