• Bittorrent Sync – synchronize folders across almost every platform and operating system
• Syncthing replaces proprietary sync and cloud services
• The Top 5 Google Drive (and not only) Clients for Linux
• De-Googling My Life – check for Google Drive replacement

Yet another popular Web-based file hosting service is Dropbox. It enables to store and share the files between users via file synchronization. There are clients for all popular OSes (Windows, Linux, MacOS).

• How to sync folders (or files) that exist outside the "My Dropbox" folder? (Dropbox Folder Sync automates the creation of symlinks).
• Selective Sync feature allows to select only subset of folders for syncing.

Other similar services:

• Mail Cloud
• Copy Cloud