DNS - Domain Name System

Bind

NSD

(nyt)

Firewall

IPtables

Built into the Linux kernel iptables provide a very flexible and powerful firewall.

Firestarter

(nyt)

Testing 

Nmap

Network Mapper.  Probes the ports on a given system to see which ones are open.  

Network Intrusion Detection System (NIDS)

Snort

Items marked with (nyt) indicate I have not yet tested it. I've heard
it's good (so it has made it on the list) but have not yet tested it
myself.