OnionShare is an open source tool for securely and anonymously sending and receiving files using Tor onion services. It works by starting a web server directly on your computer and making it accessible as an unguessable Tor web address that others can load in Tor Browser to download files from you, or upload files to you. It doesn't require setting up a separate server, using a third party file-sharing service, or even logging into an account.
Unlike services like email, Google Drive, DropBox, WeTransfer, or nearly any other way people typically send files to each other, when you use OnionShare you don't give any companies access to the files that you're sharing. So long as you share the unguessable web address in a secure way (like pasting it in an encrypted messaging app), no one but you and the person you're sharing with can access the files.
To learn how OnionShare works, what its security properties are, and how to use it, check out the wiki.
You can download OnionShare for Windows and macOS from the OnionShare website.
For macOS you can also use Homebrew:
brew cask install onionshare
For Ubuntu-like Linux distributions, you can use this PPA:
sudo add-apt-repository ppa:micahflee/ppa sudo apt install -y onionshare
OnionShare may also be available in your Linux distribution's package manager. Check this wiki page for more information.