Burp Suite is an integrated platform for attacking web applications. It contains a variety of tools with numerous interfaces between them designed to facilitate and speed up the process of attacking an application. All of the tools share the same framework for handling and displaying HTTP messages, persistence, authentication, proxies, logging, alerting and extensibility. There is a limited free version and also Burp Suite Professional ($299 per user per year).
For downloads and more information,
visit the Burp Suite homepage.
Burpsuite is a must-have tool for any Web Application penetration Tester. This should be the baseline from which you do most of your manual testing. The commecial version of the product also offers a number of features and enhancements that will make your life much easier.
also, at the time of writing, this blurb is out of date. Burpsuite is currently on version 1.4.01.