For using Dropbox as a special remote, there are currently several choices:

* rclone supports the Dropbox API,
  so the [[rclone_special_remote|special_remotes/rclone]] can be used.  
  (Actively maintained)
* Alternatively, there are some dedicated special remotes:
  * <https://github.com/TobiasTheViking/dropboxannex>  
    (Last updated 2014)
  * <https://pypi.org/project/git-annex-remote-dbx/>

At this time it's not clear which is better, so if you find one works
best, please comment below.