Nmap | ZAP | BurpSuite | Metasploite | KALI.

NMAP - "network mapper" scans network to find what's connected to it 
http://nmap.org/

Zap - web proxy and automatic vulnerability scanner
https://code.google.com/p/zaproxy/

Burp suite - Several pen testing tools (some versions are free)
http://portswigger.net/burp/

Metasploit - customizable platform for developing, testing, and using exploit code.
http://www.offensive-security.com/metasploit-unleashed/Main_Page

Kali - Linux distribution with pre-installed pen testing tools.
http://www.kali.org/

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