WinDirStat - free disk usage tool

One more excellent free tool is WinDirStat.  It shows both where the disk space is going, as well as what it is going toward, based on file type, in a clean graphical way.  In just a few clicks you can clean up gigabytes of unneeded data.  I found many .pdb files on my machine I didn't need.  Even if you have a huge hard drive, fewer files means faster defragmentation and faster virus scans.  Highly recommended.