Chatbox

Chatbox makes it easy to discuss or comment on files shared over Dropbox. Install it, right click on any files / folders inside Dropbox, and start conversations with people you shared the Dropbox folder with.

Use it for commenting and annotating files OR for simple project collaboration around folders with relevant files inside.
It works offline and syncs whenever you're next online.
Chatbox stores everything inside Dropbox. You don't need to register another account.