If you haven't seen it, there's a great RSS feed-reading tool called NetNewsWire that can read many blogs' RSS feeds, as well as other stuff like slashdot. The new version (still in beta) has support for blog editing built in, for a lot of popular blogs, including Movable Type, which is what I use.
It's pretty darn cool just as a reader, but I'm giving the editor a shot. If it's as good as the rest of the program, I expect I'll be registering it pretty soon. It's a nice bit of software.
[update] I guess it worked. =)