Wireshark is a free tool that is used for analyzing the traffic of computer networks such as Ethernet, FDDI, PPP, Token-Ring, etc. Among advantages of the tool over the similar programs are graphical user interface and various filters.
Vega Vulnerability Scanner is a free, open source tool for security testing of web software products for Windows, Linux and Macintosh operating systems. It is created by Subgraph.
Nikto Web Scanner is an open source scanner of web servers, web sites, virtual hosts. It can run numerous tests, find many types of security and server configuration problems.
ZAP or Zed Attack Proxy is a tool for finding vulnerabilities in web software products. It is designed so that not only developers and experts in security testing can use it; the tool is understandable for beginners in security testing as well.