MultCloud - Multiple Cloud Storage Manager: Migrate, move, sync, copy, backup and transfer cloud files with MultCloud, which supports Dropbox, Box, Google Drive, Mega, OneDrive and FTP, etc.

CloudGopher - Copy email, files & more across Google accounts

Koofr - Koofr offers safe EU based cloud storage with free storage space for life and options to connect multiple cloud accounts (Dropbox, Google Drive, OneDrive) and your personal storages.

odrive - odrive aggregates all cloud storage. Access, sync, share, and encrypt everything in one place. Integrations to 20+ storage services, desktop sync, Linux support, placeholder files, zero-knowledge-encryption, web client, advanced sharing, and more!

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.

CloudHQ - Integrate Box, Gmail, Salesforce, Google Drive, Dropbox, Basecamp, Evernote, and more.