FitNesse is an acceptance testing tool that allows tests to be expressed as tables of input data and expected output data, described in Fit for Developing Software: Framework for Integrated Tests.
Watir is an open source library for automating web browsers allowing automated testing of web applications. It started out on Internet Explorer on Windows but is in the process of being ported to other browsers.
Selenium is a cross-platform suite of tools to automate web application testing.
Sahi is an automation and testing tool for web applications that runs as a proxy server.
The Grinder: http://grinder.sourceforge.net/
This is an open source load testing tool in which scripts are written Jython.
This is an open source load testing tool in which scripts are written in Java.
Peach Fuzzing Platform: http://peachfuzzer.com/
Peach is a fuzzer that is capable of performing both generation- and mutation-based fuzzing.
RFuzz is a Ruby library that allows web applications to be easily fuzz tested.