MicroServer! is a portable SFTP server for Windows. It’s less than 3 MB in total, and can be run from a USB stick. It’s FREE for any use, including business and commercial.
It’s mainly suitable for one-time or sudden secure file transfer jobs from/to machines where you don’t want to install a full-featured Server!, but you still need to run such file transfers securely.
It only supports one user profile, one root folder, and incoming connections from one client at a time.
Some users may use it also to test their SFTP client software, or as a personal secure file transfer server for their small home network.