It depends on what you want to test. From a client perspective, to check that the pages are generating the correct content,
HttpUnit might suit your needs. For unit testing JSP tags that you've built,
TagUnit might give you something extra over just testing the tag handler classes with
JUnit. For more complex in-container testing of
Servlets and JSPs, try
Cactus. For performance/stress testing, check out
JMeter.
Basically, there are lots of tools but it depends on what you want to do.