Highlighting part of content is most common thing in a blog post. Especially if your are a programmer then you want to highlight the coding part. One of the best plugin to markdown content in WordPress Post is WP-Markdown. This plugin allows to publish posts using the Markdown syntax and it will convert the Markdown into HTM. Most amazing part is deactivating the plugin will not affect your highlighted posts as it stores the processed HTML.
You can use this plugin affect in posts, comments or bbPress forums. First of all get the plugin and activate it. After activating go to Settings->Writing menu and set the parameter:
After saving the settings you will see new option in post editor:
This is the output sample: