Zed Attack Proxy (ZAP) is a free and open source web application security tool that can be used to test for vulnerabilities during the development and testing phases. It includes features like an intercepting proxy, spidering to discover hidden links, both active and passive scanning to detect vulnerabilities, and reporting of results. ZAP allows users to intercept web traffic, modify requests and responses, scan sites for issues like XSS and SQLi, analyze results, and generate detailed vulnerability reports.