When working on my Mac, I’ve been using Ecto as my blog editor of choice. It gives me a lot of flexibility, enables me to write the drafts of my blog posts while off-line (I find airplanes a great place to write entries) and integrates nicely with Apple iPhoto. You can also customize it greatly and automated many tasks and repetitive HTML snippets quickly.
On the Windows front, I have also used Ecto for a while. While it works almost as well on Windows than on MacOS, except for the iPhoto integration obviously, you can feel that this is not a native application. Listening to episode 42 of Windows Weekly, I wanted to know more about the Windows Live applications from Microsoft, and especially the one to write blog posts: Windows Live Writer. And I have to say: Microsoft, great job!