Missing-E - Has quite a few features (some of which are linked individually below) and is an extremely handy add-on.
Ask box on Dashboard - Lets you send messages without leaving the dashboard.
Wrap Around Tags - Let’s you see all your tags on a post on the dashboard.
Upload Photo to All Posts - Adds a “+upload photo” to all posts.
Reply Replies - Let’s you reply to peoples replies on posts.
X-Preview - Preview those photo-post notifications without opening thousands of new tabs.