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.

Of course, you get to choose which language you develop your tests in: Ruby, Java, Python, C#, PHP, ...

Last but not least, Selenium Grid is easy to use and simple to install.

Here is a view of a hypothetical deployment architecture....

Notice that you can choose the number of nodes, the versions of each operating system within type  ... Mac OS Tiger and Leopard can be nodes in the same farm. Similarly Windows XP, Vista and Server 2003 can be nodes in the same farm.