Port Scanning

As the name suggest, these programs r used to scan certain ports or a range of ports before starting attack.This methods is used  to find which all services running  on the target system....Services include  finger,telnet or any other services.These r programmed in such a way that It could query these services to find which version of that software is running.As some software might have some bugs which could be exploited by the attacker to compromise the system.....

Both these tools r used for Auditing security in various organizations also.These tools help Administrators to find security loop holese within their networks & rectify it...

some of the popular port scanner which r available free of cost

Nesus
This program can be downloaded from www.nessus.org.It has database of bugs & their exploits.It can determine the host operating system.Hey,never use this to attack someone or u might land into jail & than don't call us to rescue u.This is for Educational purpose only.

Nmap
This tool can be used to find which all services r running on the target system including application names & their versions.This can be downloaded from www.insecure.org/nmap.This could even tell which all software r used for defense purposes at the target system.