Watcher is a plug-in for Eric Lawrence s Fiddler proxy aimed at helping developers and testers find security issues in their web-apps fast and effortlessly. Because it works passively at runtime, you have to drive it by opening a browser and cruising through your web-app as an end user. For the developer, the tool can provide a quick sanity check, so you can find problems and hot-spots that warrant further attention. In the hands of a pen-tester it can assist in finding issues that lead to other attacks like XSS and CSRF.
Source: https://threatpost.com/watcher-new-web-security-testing-tool-042009/72591/