Post authoring just became a heap easier with the Markdown plugin for WordPress. Now I can write posts quickly and easily using both HTML and the Markdown syntax. You will need to deactivate the TinyMCE visual editor, but that’s okay because it sucks anyway. If you think that being able to use Markdown in your WordPress posts and pages, you can download the PHP Markdown plugin from http://michelf.com/projects/php-markdown. I think that Markdown should be included with WordPress by default.
Update: PHP Markdown hasn’t been updated in a while and was causing problems with other plugins, so I’m now using WP-Markdown instead.