hopli
one file, no installer, no account
Run it on the computer you want to send files to and from. It shows a code; your phone scans it.
Nothing is installed, nothing is registered, and no file you send passes through a server. Delete the one file and it is gone.
Download
Other platforms
Check what you downloaded
Every build's SHA-256 is listed above. If you would rather not take our word for the file being the file, compare it:
certutil -hashfile hopli.exe SHA256
or on macOS and Linux, shasum -a 256 hopli.
The first-run warning
These builds are not code-signed, so Windows will say “Windows protected your PC” and macOS will say the developer cannot be verified. That is the operating system telling you the truth: nobody has paid a certificate authority to vouch for this file. On Windows choose More info → Run anyway; on macOS right-click the file and choose Open. If you would rather not, the checksum above is the other way to know what you have.