New note for things like DropBox, Google Drive, OneDrive, Sync.com, NextCloud, OwnCloud, etc.
NextCloud is OSS. Has permissions. Is a fork from OwnCloud.
OwnCloud - I've used portions. It has a command line client for synchronization management (very cool if you need it).
To Try: https://freefilesync.org/ - backups + sync - open source - Hacker News - https://news.ycombinator.com/item?id=29661539
Clientless, web based, cross platform, similar to Airdrop:
https://snapdrop.net/
It automagically works on a LAN. Somehow registers by IP address or something.