ProFTPD is a highly configurable GPL-licensed FTP server

ProFTPD grew out of the desire to have a secure and configurable
FTP server, and out of a significant admiration of the Apache
web server.

When the Project began, the most commonly used server was wu-ftpd.
While wu-ftpd provides excellent performance and is generally a
good product, it lacks numerous features found in newer Win32 FTP
servers and has a poor security history.
Many people, including the developers who work on ProFTPD, had
spent a great deal of time fixing bugs and hacking features into
wu-ftpd.
Unfortunately, it quickly became clear that a complete redesign
was necessary in order to implement the configurability and
features desired.

ProFTPD is not a hack based on any other server, it's an independent
source tree from the ground up.