Watcher is a Fiddler addon which aims to assist penetration testers in passively finding Web-application vulnerabilities. It automatically detects high profile vulnerabilities such as Cross site Scripting (XSS) and Open redirects. Fiddler acts as a proxy between your computer and your internet. It comes with lots of different features, however i won’t talk about other features of fiddler as it’s not a part of the scope of this article. If you would like to learn more about capabilities of Fiddler, i would recommend you reading the book.”]
Source: http://www.rafayhackingarticles.net/2012/10/detecting-cross-site-scripting-xss.html