Web Test Automation Tools

HtmlUnit - A Java unit testing framework for testing Web based applications

Selenium - Cross Browser

Watij - IE only

Sahi - An automation and testing tool for web applications, with the facility to record and playback scripts

Canoo WebTest - A free Open Source tool for automated testing of web applications in a very effective way.

JMeter - Apache JMeter is a 100% pure Java desktop application

LoadSim - A web application load simulator

Groovy - Potentially in conjunction with a specialist HTML parser to parse HTML pages as if they were XML

References:

Open Source Web Testing Tools in Java -
http://java-source.net/open-source/web-testing-tools

Watij and Selenium -
http://www.suryasuravarapu.com/2007/10/watij-and-selenium-are-they-ready-for.html