The Grinder is a Java? load-testing framework that is freely available under a BSD-style open-source license.
http://grinder.sourceforge.net/
There is also a book on performance testing using this: ISBN: 1904284000 (rather nicely one of the references in the book is this Wiki :)
Version 3 is currently in beta and uses scripts written in JythonLanguage rather than in the property file based language it previously used.
It's functionally equivalent to Apache JMeter, although JMeter is prettier and more user-friendly. JMeter is also more powerful than Grinder2 when it comes to putting together complex tests involving authentication and cookies.
Disagreement about attractiveness of JMeter over The Grinder 3: http://www.gashalot.com/blog/archives/000002.html
-- PhilAston