I love Editorial Calendar. Instead of organizing your posts in list form, as WordPress defaults to, Editorial Calendar shows them in a monthly calendar format. You can quickly drag-and-drop blog posts to different dates.
I manage a lot of our clients’ blogs, and this plug-in is a lifesaver for me.
I also like Viper’s Video Quicktags — makes it super-easy to post videos from YouTube, Vimeo, and many other services without messing with embed code.