TextMate. Once upon a time, TextMate ($55, OS X 10.7 and higher) was all the rage among the cool kids who wrote Ruby on Rails on their MacBooks while sitting at tables in college cafés. TextMate has since become less prominent as it fell into neglect and Sublime Text gained popularity, but with version 2.0 the product could be ready to make a comeback.
Markdown support is provided in one of the in-package bundles. It includes a document preview function, a Markdown cheat sheet, and lots of shortcuts for generating Markdown mark-up.
Although it isn't documented yet, the Git bundle works well to integrate TextMate with Git and GitHub. TextMate recognized my existing Git repositories without any prompting when I opened them as project directories, and it was able to update them from GitHub using a pull command from the bundle. The SQL bundle lets you work with MySQL and PostgreSQL databases.
As far as speed goes, TextMate is right up there with Sublime Text. It might be a hair slower, but I'd need a stopwatch to be sure. I certainly can't complain about it.
It looks like TextMate is coming back from its temporary obscurity. I wish it well.