Last time I tried a standalone posting app, it was Ecto, which didn't like Wordpress too much.
This time, I'm trying MarsEdit, and have applied a patch to the Wordpress XMLRPC connection by Johann Richard at 'Take the Red Pill'. This patch may behave better with Ecto, as well...
I'm pretty impressed with MarsEdit. Looks like Ranchero is doing an awesome job at avoiding bloat, producing excellent task-oriented applications.