Thumper - web application performance testing
Over the years I've been involved with writing a number of web applications. Some I am proud of, some I never want to see again. During each project, testing of the application's performance, stability under load, and regression testing of user journeys always comes up. There are a number of great bits of hardware (eg. Avalanche), and software (eg. Grinder) that help you test applications. I've even had a fun time using a room full of Grinder clients to test web services for a client - a great way to run a denial of service attack :-)...