This article is work in progress

Commercial

 ViceVersa

 http://www.tgrmn.com/

Allway Sync™

http://www.allwaysync.com/

Super Flexible File Synchronizer

 http://www.superflexible.com/

GoodSync Pro

http://www.goodsync.com/

SynchronEX

http://www.xellsoft.com/SynchronEX.html

Freeware (OpenSource)

Unison

http://www.cis.upenn.edu/~bcpierce/unison/

Foldershare (Microsoft)

https://www.foldershare.com/

Reconcile

http://www.merl.com/reports/docs/TR99-14.pdf