We've recently launched a new testing server test.rebex.net. It
supports both FTP/SSL and SFTP protocols, but still in read only mode
(for the reasons Lukas mentions above).
We run an open FTP/FTPS server <ftp.rebex.net (username: "anonymous", password: "guest"), but it only supports downloading as well at the moment (we don't have any SFTP server, unfortunately).
To support upload as well, we would have to add some barriers against people misusing the server for their own purposes, which is more complicated than it sounds.
So if you need a test server, my recommendation would be to install FileZilla Server for FTP and FTPS. It's free and good enough. For SFTP, the situation is more complicated. Windows ports of OpenSSH (supports SFTP) are free, but hard to maintain and often outdated. Bitvise WinSSHD is free, but only for non-commercial use. Another alternative is SFTP Server Lite, an experimental .NET SFTP server library - it's extremely simple to use, but it's not a stand-alone SFTP server.