Modern FTP server packages that remain in common use as of 2015 include FileZilla Server, Cerberus FTP, SolarWinds TFTP, Rhino Serv-U and CrushFTP. The FTP protocol was first developed in the 1970s, so there are many obsolete FTP server programs.
FTP server packages are similar in terms of their purpose but can be categorized by the types of FTP connection they support. There are three types: unencrypted FTP, deprecated FTP/S that uses implicit SSL/TLS, and relatively modern FTP/ES that uses explicit SSL/TLS. Modern FTP servers often support all connection types.
Since the FTP protocol is well established and widespread, modern FTP server packages are available across most platforms; even Android has FTP server software, such as the "FTPServer" app.