one to one peer to peer file transfer: justbeamit.com peer to peer file transfer (link doesn't die after one person) file.pizza

but ironically every single OS today ships with a remote desktop functionality. some even on by default.

Actually, there is: if both users are on gmail, you can attach big files to your mail, and it's automatically uploaded on Drive (and retrieved seamlessly by recipient from mail). If recipient is not using gmail, you can still use Drive to upload your file and share it. And obviously, there are many Drive competitors, like dropbox and Box, to name the biggest ones.