nmap

Nmap is a utility for port scanning large networks, although it works fine for single hosts. The guiding philosophy for the creation of nmap was TMTOWTDI (There's More Than One Way To Do It). This is the Perl slogan, but it is equally applicable to scanners. Sometimes you need speed, other times you may need stealth. In some cases, bypassing firewalls may be required. Not to mention the fact that you may want to scan different protocols (UDP, TCP, ICMP, etc.). You just can't do all this with one scanning mode. And you don't want to have 10 different scanners around, all with different interfaces and capabilities. Thus virtually every scanning technique are incorporated into nmap.