Wireshark = Network Protocol analyzer.
Kismet = A powerful wireless sniffer.
Tcpdump = The classic sniffer for network monitoring and data acquisition, I use it regularly.
Pound = The Pound program is a reverse proxy, load balancer and HTTPS front-end for Web server(s).
Trac = Project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.
Request Tracker = A free web and email-based bug tracking and trouble ticketing system. Features list, documentation screen shots, and download.
Vmstat = The command vmstat reports information about processes, memory, paging, block IO, traps, and cpu activity.
Iptraf = The iptraf command is interactive colorful IP LAN monitor. It is an ncurses-based IP LAN monitor that generates various network statistics including TCP info, UDP counts, ICMP and OSPF information, Ethernet load info, node stats, IP checksum errors, and others.
mc = Visual shell for Unix-like systems.
Postfixadmin = Postfix Admin is a web based interface used to manage mailboxes, virtual domains and aliases. It also features support for vacation/out-of-the-office messages.
pwgen - Automatic Password generation.
Linuxconf = Linuxconf comes with Mandrake Linux and Red Hat Linux, but is also available for most modern Linux distributions. You've probably encountered this tool before if you use one of these distributions, either as the whole package or in one of its modular components. Multiple interfaces for Linuxconf have been available for years, but now we're up to four: GUI, Web, command-line and ncurses.
Webmin = Webmin comes with, and was recently acquired by, Caldera Linux. This tool is not only available for most modern Linux distributions, it also runs on most major flavors of UNIX and is available in around twenty languages (though some modules are not available in all of the languages). As you might guess, Webmin is purely a web-based application and a heavily modular one at that.
OpenVPN = SSL/TLS based user-space VPN. Supports Linux, Solaris, OpenBSD, FreeBSD, NetBSD, Mac OS X, and Windows 2000/XP.