Lot's of functionality:
- https://github.com/plasticboy/vim-markdown
- https://github.com/tpope/vim-markdown. Default on Vim 7.4.
Markdown extensions:
- https://github.com/vim-pandoc/vim-pandoc-syntax
- https://github.com/PProvost/vim-markdown-jekyll
Specific functionality:
- https://github.com/nelstrom/vim-markdown-folding
- https://github.com/jtratner/vim-flavored-markdown fenced code block on top of Tim Pope
Previewers:
- https://github.com/greyblake/vim-preview, multiple input types