Markdown Mode provides a major mode for editing Markdown-formatted text files.
This package is not currently part of the Emacs distribution.
Live preview as you type
There are a few possibilities.
See impatient-mode (in MELPA). It only depends on emacs packages. This package is designed for html though, but it is possible to make it render markdown. Evaluate this somewhere so you have the function:
defun markdown-html (buffer) (princ (with-current-buffer buffer (format "<!DOCTYPE html><html><title>Impatient Markdown</title><xmp theme=\"united\" style=\"display:none;\"> %s </xmp><script src=\"http://strapdownjs.com/v/0.2/strapdown.js\"></script></html>" (buffer-substring-no-properties (point-min) (point-max)))) (current-buffer)))
Then M-x imp-set-user-filter in your markdown buffer, and supply markdown-html when it asks for the function. Live updating markdown!
livedown is specific to markdown but it requires nodejs packages (to be easily installed with npm).