ToffeeShare is incredibly useful, but AFAIK it is not FOSS. You could alternatively use something such as Send (a fork of the now dead Firefox Send project), which has quite a few public instances [0]. However, I've been personally liking a lot ShareDrop [1], which is open-source and has an official public instance. In ShareDrop data transfer is done over WebRTC, and the server is only responsible for connecting the two machines, whereas in ToffeShare and I believe Send the files pass through their servers, although encrypted.

[0]: https://github.com/timvisee/send-instances/

[1]: https://github.com/szimek/sharedrop