SimpleSSHD is a free (open source, no ads) SSH server based on dropbear that supports scp, sftp, and rsync. It only supports public-key based authentication (no password/interactive auth). It does not use root, which means it must listen on a port over 1024 (defaults to port 2222).
New approach to accessing /sdcard. Automatically re-start on package upgrade if Start on Boot enabled. Fix "Non-matching signing type" error with older ssh clients.
If you have problems with external storage, try "Enable /sdcard" menu option.