bftpd is an FTP server for Linux, BSD/OS, FreeBSD, Solaris, DG-UX and Tru64. (I don't know if it runs on other systems, please mail me if you have tried it). It runs either with inetd or standalone.
It tries to be very configurable while being fast and small. You can make defaults for each configuration option, and then override these defaults in user-specific and directory-specific structures.
Features of bftpd include: