The Qumana blog editor is a free cross-platform blogging client with support for a variety of blogging platforms including Wordpress, Blogger, MoveableType and others. It offers WYSIWYG editing, offline composing, and a variety of other features. Setting it up to work with a public blog is quite easy, requiring you just to enter the blog's root URL. It will then prompt you for your username and password and you're pretty much set.
Unfortunately, there are two catches. First, it appears to be a java application with all that entails for resource usage and UI inconsistency. Second, it's free because Qumana wants you to use their Q Ads service (which is built into the editor). Q Ads are text advertisements that you can manually add to your blog (by defining keywords) and Qumana will share some of the revenue with you. Fortunately it appears you can use the Qumana editor even without inserting Q Ads.
It's not as Mac-like as MarsEdit by a long shot, but if you'd like to try a free editor that works easily with a variety of blogging platforms then Qumana is worth a look. The Qumana blog editor is a free download from Qumana.