jWebUnit is a Java framework that facilitates creation of acceptance tests for web applications. It evolved from a project where we were using HttpUnit and JUnit to create acceptance tests.

Open source Java API for testing web sites without a browser.

HtmlUnit is a java unit testing framework for testing web based applications. It is similar in concept to httpunit but is very different in implementation. Which one is better for you depends on how you like to write your tests. HttpUnit models the http protocol so you deal with request and response objects. HtmlUnit on the other hand, models the returned document so that you deal with pages and forms and tables.

Apache JMeter is a 100% pure Java desktop application designed to load test functional behavior and measure performance. It was originally designed for testing Web Applications but has since expanded to other test functions.

MaxQ is a free web functional testing tool. It includes an HTTP proxy that records your test script, and a command line utility that can be used to playback tests. The proxy recorder automatically stores variables posted to forms, so you don't have to write that stuff by hand.