Windows only: Free utility Disk Space Fan analyses your hard drive usage to help you determine what's taking up space on your hard drive, representing it all with fancy visualisation eye candy.
Disk Space Fan joins a long line of disk visualisation tools, from the classic WinDirStat to DriveSpacio, Disktective, and others. WinDirStat has always had a special place in our hearts when it came time to visualise our hard drive usage and knock off space-wasting files, but it's not the most attractive tool in the toolbox. Disk Space Fan does the same sort of disk analysis, but it's also very attractive in addition to being very functional.
Eye candy aside, it's scans are fast, and the Explorer integration makes it open up any file or folder for a closer look. You can click any slice to drill down further (and yes, it has fancy animated transitions when you do the clicking), and you can open or delete any file or folder directly from the app.
Disk Space Fan is a free download, Windows only. A Pro version is available with a few more features, but the core that makes it great is available for free.