If you're a Mac-owning WordPress user, there's a good chance you're also a MarsEdit user. If so, I doubt you're in the market for any other blogging software, but if you are (or if you're looking for a cheaper option), you might check out newcomer Metropolis.

It's got a lot of the same functionality as MarsEdit, such as multi-blog support, HTML and Rich Text editing, customizable post previews, offline drafts, and image insertion, but adds a couple features of its own:

• Twitter integration — Allows you to auto-tweet any post you publish and also track how many other people are tweeting about it. Also lets you run Twitter giveaways by generating a list of random names from the list of people who tweeted any given link to your site.
• Comment management — View comments and reply if desired, approve/delete/edit comments, Notification Center alerts for new comments, and Gravatar support.

For $20, this looks like a pretty good option. Just keep in mind that, unlike MarsEdit, this is ONLY for WordPress. There is currently no support for other platforms like TypePad, Squarespace, Movable Type, etc.