gnome-blog is a GNOME applet that can post to weblogs using several API. The manual didn’t include WordPress.com to the list of supporte sites, but it seems to work just fine; this entry is posted from gnome-blog.
It’s only provide minimal feature though. There’s only support for several markup (bold & italic font, adding link). Might be useful for low bandwidth situation (which is quite the norm for me 😦 )