For the dedicated blogger, you can't get better than MarsEdit. Originally written to work hand-in-hand with NetNewsWire, it's a tiny text editor with simple tag colouring features and little in the way of assistance when it comes to entering code.
It is compatible with a wide range of blogging tools, including Wordpress, MovableType and Blogger, from which it can drag in categories and other relevant settings. By giving it your username and password, you can use it to post straight into your blog's database, and upload images to your server space without ever having to navigate the online entry forms.
If you think that sounds like a slim benefit barely worthy of the asking price, download a trial and give it a go. You will never turn back. Combine it with the likes of free ImageWell (xtralean.com) for shrinking your pictures and you have the best blog-writing, two-piece going.
Since we made the switch, we only use Wordpress' online interface for system configuration, rearranging our widgets and configuring plugins. As a result, our blogging is both more focused and more frequent.