ProFTPD is a high-performance, extremely configurable, and most of all a secure FTP server, featuring Apache-like configuration and blazing performance.
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.
In addition to wu-ftpd, there are a few of other FTP servers available which are designed to be light-weight and secure at the expense of configurability. For example, Troll FTP is an excellent FTP daemon which is considerably more secure and less resource-intensive than wu-ftpd. Unfortunately, while it is quite suitable for basic FTP services, it does not offer the feature set required for more sophisticated FTP sites.
ProFTPD is not a hack based on any other server, it's an independent source tree from the ground up. A number of well known and high traffic sites use ProFTPD.