This is just a short list. Add or comment what you think is appropriate for this thread. Reviews are welcome.

Web Vulnerability Scanners:
Burp Suite https://portswigger.net/burp/
WebScarab https://www.owasp.org/index.php/Category:OWASP_WebScarab_Project
SQLmap http://sqlmap.org/
Open SSH http://www.openssh.com/
ZAPP https://www.owasp.org/index.php/OWASP_Zed_Attack_Proxy_Project

Sniffers:
Wireshark https://www.wireshark.org/

Wireless:
Aircrack http://www.aircrack-ng.org/

Security scanner:
Nmap https://nmap.org/download.html