• Google Browser Sync
  Google Browser Sync for Firefox is an extension that continuously synchronizes your browser settings – including bookmarks, history, persistent cookies, and saved passwords – across your computers. It also allows you to restore open tabs and windows across different machines and browser sessions
  in Public bookmarks with google browser sync firefox extension continuously synchronizes settings by 15 users
• PowerFolder - Wikipedia, the free encyclopedia
  PowerFolder is an free, open source program that synchronizes files and folders over the internet or a LAN. For this program to work, it must be installed on all computers that will share the files.Although PowerFolder is open source, only the paid version has secure file transfers and an unlimited amount of folders. The free version only supports 3 folders, 10 GB of files on each folder, does not include HTTP tunnels, cannot run as a system service, or have a webadmin.
  in Public bookmarks with files folders free open powerfolder program source synchronizes by 2 users
• Unison File Synchronizer
  Unison synchronizes files and directories stored on different hosts on the Internet. It can handle changes made to either replica of a distributed directory structure, and hence is more than just a mirroring utility. It is open source, distributed under the GNU GPL, and works on Windows, Linux, Solaris, OS X, etc
  in backups windows with backups windows unison synchronizes files directories stored hosts by 12 users

synchronizes from all users