Proftpd - nice daemon, easy to get going on Debian, secure
wu-ftpd: older, has a bad track record for security holes
pureftpd - newer, very secure, works with with LDAP and ISPMan
AnomicFTPD (http://www.anomic.de/AnomicFTPServer/) - a small, fast, free/GPL, "full" (RFC959 compliant) terminal-based ftpd server in Java with own user account management.