trying out MarsEdit, by the guys who brought you NetNewsWire (new NNW 2.0 out now too)
pretty good. easy to use. doesn't seem to do categories yet, though.yes it does, initially it doesn't quite know your settings (eg categories), you have to give it a username/password before it can "know" your blog. try choosing your blog and hitting "refresh" .. should do this automatically, methinks.
also would be nice if it automatically found your stylesheet for the preview. a la kung-log
these guys are good. they give out code, too. helping me help you.
Currently playing in iTunes: Worried Eyes by Eagle-Eye Cherry