Selenium Grid is a tool that dramatically speeds up functional testing of web-apps by leveraging your existing computing infrastructure. It allows you to easily run multiple tests in parallel, on multiple machines, in an heterogeneous enviroment.
Based on the excellent Selenium web testing tool, Selenium Grid allows you to run multiple instances of Selenium Remote Control in parallel. Even better, it makes all these Selenium Remote Controls appear as a single one, so your tests do not have to worry about the actual infrastructure. Selenium Grid cuts down on the time required to run a Selenium test suite to a fraction of the time that a single instance of Selenium instance would take to run.