Best there is

Ive been blogging for 4 years and tried various methods, and usually Ive ended up just using the online WordPress interface. The other Mac blogging clients just dont offer enough options to help you out; you still have to go online and tweak it. MarsEdit is fast and lets you fire off blog posts that are ready to go. It even has a browser bookmarklet if something inspires you. One star withheld because it has some glitches with formatting text around images.

Notratched about MarsEdit - for WordPress, Tumblr, Blogger, etc.