https://syncthing.net/Because if I've only just discovered it, there's probably someone else whose life can be enriched by it too.
It's a file-synchronisation tool. Think Dropbox/OneDrive without the cloud storage, rather than arcane rsync voodoo. Once set up it appears to Just Work™ (and when it comes to syncing the photos on my phone with my desktop PC, a whole lot faster and more reliably than Dropbox ever managed).
