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