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 :)
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Version 3 is currently in beta and uses scripts written in JythonLanguage rather than in the property file based language it previously used.
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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.
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Disagreement about attractiveness of JMeter over The Grinder 3: http://www.gashalot.com/blog/archives/000002.html

-- PhilAston