Editoria/markdown_npp provides a good set of language definitions for Markdown.

These instructions are adapted from this article (originally posted in the Notepad++ forums). Primarily, I've replaced the Python script with a pandoc command.

1. Install the PreviewHTML plugin from the web site - Plugin Manager has an old version.
   1. Copy the zip file contents to Notepad++\plugins
   2. If you put the DLL in <user>\AppData\Roaming\Notepad++\plugins you'll also need to allow AppData plugins by adding an empty allowAppDataPlugins.xml file to %ProgramFiles(x86)%\Notepad++.
2. Install pandoc for converting from Markdown to HTML. I recommend using Chocolatey
3. Configure PreviewHTML
   1. Plugins → Preview HTML → Edit filter definitions (this opens Filters.samples.ini)
   2. Add the following section, then Save As Filters.ini:

[Markdown]
Extension=.md .markdown
Language=Markdown
Command=pandoc -f markdown -t html "%1"

See this post (alternate link)) for more configuration options like context menus, hot keys, etc.