I'm one of those weird users who likes paying for good software, but there's nothing to buy that I like. (I paid for BlogJet 2.0 and I'm not a happy customer just now.)
So I'm interested in this Firefox extension: ScribeFire (previously Performancing for Firefox). As an extension it should work for OS X and XP. I'll update this post with my experience.
 In particular they expect that the user always posts from one machine that holds a database of posts. Sorry, doesn't work like that.
Update: I thought it was double posting on edits, but I was ignoring the 'post as edit' button. That did the trick. So far it works well for OS X and blogger. I'm very interested and will post more.