Reader Shawn Blanc pointed me to a great post of his covering ecto, a popular desktop blog publishing client for OS X. Shawn goes step by step through using ecto and explains the features. Personally, I don't use anything to supplement my blogging and just write directly inside of the WordPress admin panel as I thoroughly enjoy the accurate post preview.
What do you use when blogging? I can't live without my dictionary widget and capture widget - although I have been playing around with Skitch for screenshots. When it comes to posting web-friendly code, nothing beats postable.