There are many different FTP server software programs for Linux, most of which are included in your distributions software repositories. Different FTP servers have different strengths, weaknesses and design goals.
vsftpd claims to be the fastest, most secure FTP server for Unix-like operating systems. vsftpd touts its lightness and stability. While vsftpd doesn't have every advanced feature found in FTP servers that may be slower and larger, it has many configuration options and features.
Some included features include IPv6 support, SSL encryption and bandwidth throttling. vsftpd includes virtual user support with many per-user configuration options, as well as per-IP address configuration options and limits. vsftpd uses a single configuration file with a simple format.
vsftpd is free, GPL-licensed software and is included in most Linux distribution's repositories. If not, you can download it here.
Pure-FTPd is a simple, secure, highly-flexible FTP server. Users can install Pure-FTPd and launch it immediately without editing a single configuration file; Pure-FTPd is controlled with command-line switches. It has many security features, not least of which is the ability to run as a limited user without root privileged.
Pure-FTPd is focused on standards-compliance and security, and runs on Linux as well as many other Unix-like operating systems. Its support for quotas and multiple authentication methods is targeted at pleasing FTP hosting providers.
Pure-FTPd is open-source software released under the GPL license. Install Pure-FTPd from your Linux distribution's repositories, or download it from here.
ProFTPD is a secure FTP server with a focus on configurability. Unlike vsftpd and Pure-FTPd, ProFTPD's priority is exposing a large amount of configuration options to users. Users who want more configurability than other FTP servers allow should install and run ProFTPD. System administrators familiar with Apache Web server configuration should feel right at home with ProFTPD, which was inspired by the Apache HTTP server.
ProFTPD has a single configuration file containing its many options, which aren't split up and divided among many files. It also supports ".ftpaccess" files for per-directory permissions, similar to the Apache Web server's ".httpaccess" files. ProFTPD can be extended with "modules" for encryption, other authentication methods and more.
Pure-FTPd is free, GPL-licensed software. Install Pure-FTPd from your Linux distributions package manger, or download it here.
iFTPd, or Independent FTP Daemon, is different from other FTP server software. Most FTP servers install themselves on a machine as a system daemon; iFTPd is different. iFTPd is a standalone FTP server daemon that doesn't need to be installed and configured on your system, you can just run it and share files.
iFTPd can also be setup and used by users who don't have root permissions on their current machine, and doesn't have any dependencies that must be installed.
iFTPd is open-source software and functions on any platform with a Java virtual machine. Download iFTPd here.