NOTE: If you choose the GitHub API for conversion (set parser: github in your settings), your code will be sent through https to github for live conversion. You'll have Github flavored markdown, syntax highlighting and EMOJI support for free ❤️ :octocat: 🎁. If you make more than 60 calls a day, be sure to set your GitHub API key in the settings :)

LINUX users: If you want to use GitHub API for conversion, you'll need to have a custom Python install that includes python-ssl as its not built in the Sublime Text 2 Linux package. see @dusteye comment. If you use a custom window manager, also be sure to set a BROWSER environnement variable. see @PPvG comments

Features :

Markdown conversion via builtin Markdown Parser (python-markdown2) or via Github API : just choose in your settings.
Browser preview auto reload on save if you have the ST2 LiveReload plugin installed.
Builtin parser : Support TOC, footnotes markdown extensions
CSS overriding if you need
YAML support thanks to @tommi
Clipboard selection and copy to clipboard thanks to @hexatrope

Installation :

you should use sublime package manager
use cmd+shift+P then Package Control: Install Package
look for Markdown Preview and install it.

Usage :

optionnaly select some of your markdown for conversion
use cmd+shift+P then Markdown Preview to launch a preview
or bind some key in your user key binding, using a line like this one: { "keys": ["alt+m"], "command": "markdown_preview", "args": {"target": "browser"} },
once converted a first time, the output HTML will be updated on each file save (with LiveReload plugin)

Uses :

python-markdown2 for markdown parsing OR the GitHub markdown API.

Licence :

The code is available at github https://github.com/revolunet/sublimetext-markdown-preview under MIT licence : http://revolunet.mit-license.org