CloudSync!

Syncthing replaces proprietary sync and cloud services with something open, trustworthy and decentralized. Your data is your data alone and you deserve to choose where it is stored, if it is shared with some third party and how it's transmitted over the Internet.

Cyberduck is an open source FTP client.

odrive is a free application that makes it easy to manage, access, and share all of your files from one place. Integration into more than 20 services, full bi-directional desktop sync, placeholder files, a robust web client, and advanced sharing capabilities.

GoodSync is the high-end professional easy and reliable file backup and file synchronization software.

Air Explorer is an application that manages easily all your files in cloud servers (Google Drive, OneDrive, Dropbox, Mega, Box, Mediafire, WebDAV, Copy, Yandex,...) and FTP and SFTP services.It transfers files between the clouds, synchronizes folders between any cloud or your computer and, optionally can encrypt your files when you upload them to the cloud. Also, you can schedule your file transfers and synchronizations.Moreover, it supports multiple accounts from the same server and centralizes the search of your files across all the clouds.Air Explorer makes all your clouds work as one, allows drag'n drop files between your computer and the clouds and, copy/paste between cloud servers and pause and continue uploads and downloads.It doesn't lose the clouds features like thumbnail view of the pictures and sharing files.It works with a plugins system to add cloud servers. Air Explorer can manage your files using the command line. »

Cloud storage as hard disk

Insync is a Google Drive sync and backup client with multiple account support.

ExpanDrive is a Network file system client for Mac OS X and Microsoft Windows that facilitates mapping of local volume to any SFTP server the user can access. It also can connect to FTP, Amazon S3, RackSpace Cloud Files, OpenStack Swift Object Storage, DreamObjects, WebDAV, Dropbox, Google Drive and Box.com. When a server is mounted with ExpanDrive any program can read, write, and manage remote files (that is, files that only exist on the server) as if they were stored locally.

Sync & share your files easily & securely with 256 bit AES end-to-end encryption, using the award-winning TeamDrive utilizing cloud storage or your own server!

Unlike a typical file transfer client, WebDrive allows you to open and edit server-based files without the additional step of downloading the file. Using a simple wizard, you assign a network drive letter to any FTP, SFTP or WebDAV Servers. By connecting through a virtual drive, there is no need to learn a separate client interface. You access and edit files on the server the same way that you interact with files on your local PC. WebDrive can also map a drive letter to corporate SharePoint servers, as well as Amazon S3, Google Drive and DropBox. »

Cloudsfer is a cloud data migration system that enables you to transfer your personal content and files between such cloud storage providers and accounts as DropBox, GDrive, Box, OneDrive, Facebook, Instagram and so on.Cloudsfer gives you the flexibility to manage your cloud data, without ever having to pay for extra space. Offering an optimal solution for seamless data transfer and back-up, Cloudsfer migrates data between cloud services, meaning that you no longer need to worry about your DropBox space dwindling, or having to back up your Facebook, Instagram or iCloud. Even if you have hundreds or even thousands of files to move, Cloudfer allows you to easily migrate your content between all cloud-based and on-premises platforms.Finally, you are in full control of your content. »

It's free app, which can combine cloud drives for Dropbox, Box, Google Docs, SkyDrive, SugarSync, FTP, etc, so that you can merge them together and centralize management.

CloudMerge is a new cloud storage merging platform similar to ODrive, MultCloud etc. that allows it’s users to combine different cloud storage drives, e.g. Google Drive and Dropbox [For now only these two are being listed], into one big universal drive. The following are the list of features that CloudMerge provides:

Are you looking for an app that lets you add multiple Dropbox, Box, SugarSync, GoogleDrive, SkyDrive, WebDAV accounts? Then you come to the right place. AirFile is the only one app that allows you to connect to multiple cloud accounts and transfer files between them easily.