I just link the Nexus via USB and copy the files I want over but the link I found looks good. No sync app available as it's easy to transfer files to the PC. Airdroid might work for other android machines as well. It's free if you use your router and works both ways. Otherwise have no suggestions, sorry.
Yes, I copy files across that way but I have an extensive photo album on my Nexus that I adjust from time to time and a sync would be useful.
Last attempt to transfer a bulk of files across to the Nexus caused the Nexus to crash. A message kept coming up on the Nexus saying Android had stopped working. Re-starting the Nexus would allow further progress but it's not happened before. It seems ok with small transfers but nothing over around 1GB. Is it the latest version of Android that's causing me the problem I wonder? As I say, my photo album is about 7GB in size and previously I've transferred the lot in one go.
If you connect your tablet to the PC by USB a folder pair can be set up in SyncToy. I use this on my tablet,which admittedly is not an nexus, but I assume if you can see your tablet folders in windows explorer the sync should work.