Port Scanners

nessus

The "Nessus" Project aims to provide to the internet community a free, powerful, up-to-date and easy to use remote security scanner.

A security scanner is a software which will audit remotely a given network and determine whether bad guys (aka 'crackers') may break into it, or misuse it in some way.

get it.

nmap

Nmap ("Network Mapper") is an open source utility for network exploration or security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts.

get it.