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.
Download and use it now: Syncplify.me MicroServer!