ZAP OverviewOWASP Zed Attack Proxy Project (ZAP) is a popular Java-based and open source security tools. It is useful for performing penetration tests on your (or ones you have permission to test) web site for security vulnerabilities.
It works similar to Fiddler, but has several tools for helping to find the vulnerabilities, not just give you the ability to hack at requests. It does this by having you change the proxy in your browser to point to ZAP and then point ZAP to your corporate proxy or the web site itself.
It is extensible via a plug-in architecture. There are lots of videos and tutorials on how to use ZAP. For more details there is an excellent ZAP Getting Started Guide that you can use to get everything installed and explains how to start using it.
Another great resource is: Getting Started with ZAP and the OWASP Top 10: Common Questions