I'd love a plugin that would allow me to cross publish my posts from specific categories into other blogs on blogger, livejournal, or other WP blogs.
My personal blog ( http://www.elainevigneault.com ) has a number of categories, some of which could/should be their own blogs. A plugin like I described would be excellent for something like that.
Another idea for WP blogs with multiple users - a plugin that lets users post to the group blog AND to their own blog at the same time.
I understand third party blogging tool would probably work for this, but these plugins would be cool, too :)