Once installed, you can locate the largest files by selecting a drive on the left-hand side, and then choosing the Largest Files tab on the bottom, which sorts by file size regardless of the directory they are located so you can quickly identify files for deletion. The Largest Folders tab shows you a view that resembles the previously mentioned Windirstat (another app that's a whiz at visualizing your hard drive usage), with the option to drill down into folders. Hidden behind the options panel you will find some even better settings—the ability to filter by file type, or only analyze files larger than a specified size, making this a great utility for cleaning up your drive.
Free Disk Analyzer is a free download for Windows users only. For a similar utility with more eye-candy, take a look at previously mentioned DriveSpacio.