The app is robust, connects seamlessly at startup and does not crash. The app can map cloud storage as a local drive as well as a folder on a local drive, thus obviating the need to have free drive letters. The app displays the amount of used and free space and makes it easy for the user to see how much storage is available on each cloud account.
GoodSync - GoodSync provides highly reliable file backup and synchronization for both individuals and businesses.
RaiDrive - Mount NAS, Router, Google Drive, OneDrive and Dropbox to a network drive or file explorer.
rsync - rsync is a file transfer program for Unix systems. rsync uses the "rsync algorithm" which provides a very fast method for bringing remote files into sync.
Mountain Duck - Cyberduck for mounting volumes in the file explorer. Available for Mac and Windows.
Syncthing - Syncthing replaces proprietary sync and cloud services with something open, trustworthy and...
Air Explorer - Air Explorer is a software to manage all your multiple cloud drives (like Dropbox, Onedrive, Google Drive, Mega, Mediafire, Box, Hidrive, Yandex, Baidu,...) as well as WebDav and FTP connections.