WU-FTPd, a replacement ftp server for Un*x systems, featuring logging of transfers & commands, on-the-fly compression and archiving, classification of users' type and locations, per class limits, per directory upload permissions, restricted guest accounts, system wide and per directory messages, directory alias, cdpath, filename filter and virtual host support.
In short, the most popular piece of r00table software available, very often preinstalled with your favourite distro, so that evil h4x0rs and script-kiddies can compromise your b0x in no time.