Sync & Backup Apps
This will be short - may expand later on. Will be mentioning 3 particular apps.
AllWay Sync
This was introduced to me by a friend (Kirk - you know who you are). For local sync (same LAN/WLAN) this works great. I've not tested it for online purposes, but it does an awesome job for localized sync. Localized means on the same LAN/WLAN, or with any available storage that you can access. It also supports some online services to make things easy. Also has MAC support. Best thing? One-time purchase.
Resilio Sync
This is something that I came across when looking for an easier way to sync stuff with various systems (PC/laptop/VPS). This does everything needed where firewall configuration and such is concerned. Works on virtually all platforms (Windows/Mac/Linux) - and has support for multiple NAS devices. As of this writing it's a one-time purchase as well.
This is coming from BTSync days if you followed that for use with SeedBoxes and such. It works using similar technology to BitTorrent for a decentralized file sharing - if that's what you're doing. If you just want to sync between your own devices, the latest Resilio Sync works great. Due to license changes, version 2.6.4 is the last one that works on a server - after this you need to get a business license to sync. That's a bit unfortunate for persons that have either physical personal servers or VPS providers that only use Windows Server. Which takes us to the last one on the list.
SyncThing
This is something I've been using for years - it works great but it's not as friendly as Resilio Sync or AllWay Sync. It gives you similar functionality to Resilio in connecting the devices - good security - but is a bit technical to set up.
Conclusion
Pick the one you want for your specific needs. If you have stuff you need synced only locally - AllWay Sync is your tool. If you need something that will constantly sync and not give problems with file duplication or versioning due to open databases - either Resilio or SyncThing will work. If you want simplicity for the latter scenario, then Resilio is for you. If you don't mind the more technical aspect - SyncThing is for you. If you need server sync then definitely go with SyncThing.
AllWay and Resilio are of the "freemium" category - they're free with limited functionality versus the paid versions, but for most usage cases you don't need more than what they offer.
SyncThing is open source and fully free - but just expect the setup and configuration to be a bit more tricky.
Knowing the solution doesn't mean knowing the method. Yet answering correctly and regurgitation are considered "learning" and "knowledge".