Syncthing

OnionShare

I don't think the entire service is open source but I really like https://wormhole.app/

pairdrop.net maybe?

It's mainly for local network, but you can do internet transfers.

https://github.com/schlagmichdoch/pairdrop#features

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SFTP should come with your OS. If you're on *nix, some fiddling around with usergroups is recommended for security reasons.

why not something as basic as sftp?

rsync

Edit: Or maybe you want a GUI?

I wrote my own. I aimed for a different UX than most services. For my use case I have a few devices that I often share files between. So opening the tool on both devices was a bit annoying. Instead you select the file on the first device and you get a push notification on the other. Then the transfer is done over WebRTC (locally if possible). All communication is done end-to-end encrypted and over your browser's push service.

Hosted: https://filepush.kevincox.ca/

Source: https://gitlab.com/kevincox/filepush

FileZilla.

Syncthing is pretty good.

In order of personal preference:

scp (sftp)

rsync/openrsync

nfs

host files on an ftp server with ftpd

hosting files on an http server with httpd

maybe 0up.io?

Depends on where you want to transfer your files. Do you want to sync your files to a server? Syncthing is perfect for that. SFTP and SCP also work well enough for that. Do you need to send a file to somebody else? Check out croc or portal.

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https://github.com/positive-intentions/chat

It uses webrtc for P2P connections.