Selenium is a test tool for web applications. Selenium tests run directly in a browser, just as real users do. And they run in Internet Explorer, Mozilla and Firefox on Windows, Linux, and Macintosh. No other test tool covers such a wide array of platforms.
(Note: some of these are older - make sure you read comments, etc. The InfoQ article above is a bit newer and references some recently released Selenium commands that make these outdated.)
Mike also sent me many of the other links here as well!