This Pressbox plugin by esharris adds a new tab to the Media Upload screen that allows you to insert images and files from Dropbox into your posts.
Additionally, the plugin provides a new shortcode that lets you add links or display images anywhere you want on your blog.
The author said: “This plugin is under heavy development so please feel free to send me feedback, bug reports, and feature requests!
Credits: This plugin bears a great debt to the work of Joe Doleson’s Wp To Twitter plugin for his excellent example of OAuth integration with WordPress.”
Requires PHP 5.2 and cURL.
Download and info: http://wordpress.org/extend/plugins/pressbox/