Thanks to Cheryl's post I reactivated my Posterous account and plan on using it going forward. I have only heard good things about the services so check it out when you have a second.
Do you have a self-hosted WordPress blog? Do you also use Posterous?
You could, of course, post to Posterous first, and have Posterous autopost the article on to your WordPress site.
But, if, for whatever reason, you prefer to work in WordPress first, POSTERIZE is the plugin in for you. It automatically forwards your WordPress posts to Posterous.
And, from Posterous, you could set the posts to go out in several more directions.