If you are having trouble running a copy of MultiMarkdown Composer downloaded here, see this page.
MultiMarkdown Composer 2.2.2 — The 2.2 release adds some small improvements, a few small fixes, and a few really big ones. Should be much more stable. See the knowledge base for more information.
MultiMarkdown Composer 2.1 beta (build 2) — This is a preview release of 2.1. It is non-sandboxed, and requires that you have a purchased copy of version 2 on your machine (at least for the first time you run a non-sandboxed build). Please be sure to run the Mac App Store copy of version 2.0 at least once before running this copy. For more information, see the knowledge base article.
MultiMarkdown QuickLook Generator — A free program from Fletcher T. Penney to preview MultiMarkdown text or OPML documents as rendered web pages. An alternate version of this program is available on github, but this version is more reliable. It uses proprietary code, so the source code is not available. More information is here. (Updated 1/7/13)
mmdc - for Composer version 2 — A command line utility that allows you to type mmdc or mmdc foo.txt to edit or create files with Composer from Terminal. (Updated 2013–01–24)
mmdc - for Composer version 1 — Installer for mmdc that works with MultiMarkdown Composer version 1.