OnionShare is an open source tool that lets you securely and anonymously share a file of any size.
"Securely and anonymously share files of any size. A web server is started, making OnionShare accessible as a Tor Onion Service, potentially temporarily or in a stealthy manner, over the Internet. An unguessable address is generated and is shared for the recipient to open in the Tor Browser to download the files. No separate server or third party file-sharing service required. You host the files on your own computer."
Developed by Micah Lee.
Open source, released under the GPLv3.