The newest version adds support for AtomPub, which like XML-RPC, is a protocol for connecting blog software to your blog itself. Daniel Jalkut, MarsEdit's developer, says to consider AtomPub support "introductory" -- but if you are using a custom blogging platform that uses AtomPub, you can now use MarsEdit to create content.
MarsEdit 2.2 also adds customizable image markup in addition to the built-in macros for image alignment. So if you frequently like to have images aligned in a certain way, you can create your own custom macro.
MarsEdit 2.2 is also significantly faster (I was pleased to see this myself, as start-up time was much quicker than the previous version). There is also now support for removing media items from the media catalog, a pull down editor for selecting the date, and improved MIME type generation for uploaded files.
Mobile fans take note -- Daniel is working on an iPhone version of MarsEdit, and it will be released as soon as he is satisfied with the quality of the final product.
MarsEdit requires Tiger or Leopard. The update is free for current users. You can try MarsEdit for free for 30-days or buy it for $29.99.