wzdftpd offers the following features (and the list is constantly changing):
Free - wzdftpd is distributed under the GPL
RFC compliant !
Server runs in user mode, thus does not need root privileges - on the other way, it is not designed to use system users permissions, it uses its own user database and permissions.
Simple configuration file, supporting inclusions
Online administration, using SITE commands
Backends to store users/groups in different structures
Modules: internal, using shared librairies, or external (scripts/applications), and protocols
Powerfull event system
Jobs scheduler, also known as crontab
Users are virtual: you do not need to create users in system
SSL/TLS support
IPv6 support (experimental)
win32: can auto-register as a service, unregister, start/stop
Server is designed to run as a non-privileged user to enforce security
Server can detect ip changes (dynamic ips) and set ip and passive ip accordingly
Cross devices operations (copy/move) (though can be slow)
Logging support (compatible with the wu-ftpd standard)
Designed for high performances: coded with C, multithreaded