FreeFileSync

Good news: as of v10.0 FreeFileSync is ad-free!

Very good tool, but shame about the (optional) OpenCandy in the installer.
You can donate to get an installer without OpenCandy adware. Unfortunately, you have to donate for every update then, but you can choose the amount yourself.

Tip to avoid malware like OpenCandy and unwanted toolbars: install Unchecky.

[Edited by RobertL87, May 14]

Good news: version 10.0, released on April 27 2018, no longer includes ads in the installer.
Previous versions used to bundle optional software that was checked by default.

I really loved this program but the malware introduced in the installer has caused me to abandon it. The last time I installed this program (on my fiancee's laptop) the installer got to the page asking to install some kind of adware. While reading through this page, after only a few seconds, the installer jumped to the next page and started installing Freefilesync (and, as I would eventually find out, the adware program I was trying to read about). My hands were not touching her laptop so I know I didn't accidentally hit a key. Just to be certain I uninstalled the program and ran the installer again. For the second time, I watched as the installer jumped to the next page after only displaying the option to opt-out the adware for a very brief few seconds. I, honestly, was not given enough time to reject the install of this adware (yontoo).

I intended to run a scan using Malwarebytes immediately after this but was in a hurry and had to leave to do something. I forgot about this issue and, within the next day, pop-ups and notifications for various obviously fake issues started displaying on her computer. I immediately ran a scan with Malwarebytes and cleaned up the mess that the adware/malware had made.

It is really sad to have to write off such a great program due to such a poor decision by the developers to support their work with malware.

[Edited by ecsa0014, January 29]

Very important Advantages of FreeFileSync (FFS):

• Triangle-sync as only Allway Sync, GoodSync, SyncLess or FreeFileSync can do.
• VSS-support (Volume Shadow Service), only with VSS-Support it is possible to sync open files
• Portable app with no need of admin rights.
  SyncLess, Synkron or Toucan are also portable, but have no VSS-Support.
• No limitation of the number of synced files as with the free version of Allway Sync

Not very important Disadvantages of FreeFileSync (FFS):

• FreeFilesync cannot synchronize WebDAV-files as only Allway Sync or GoodSync can do
• FreeFilesync cannot synchronize "Windows Mobile" files or folders as only Allway Sync or GoodSync can do

Very powerful, super easy to use and versatile. For those not used to command-line interfaces. Two ways, mirror, update and custom sync. Capable of syncing several folders at a time. I've never tested, but it is supposed to be able to sync from/to online storage via SFTP or FTP