Tumblrize posts to Tumblr whenever you hit the "publish" button. It uses Tumblr's simple API to keep posts in sync; when you edit your WordPress post, it updates your Tumblr post.
Tumblrize is very lightweight. It just requires you to input your Tumblr email and password in the plugin's options screen. After that, you're ready to cross-post!
Other options allow posting to Posterous, sending additional metadata from your WordPress entry (notably tags), to Tumblr, and more.




