WindirStat

I use a great utility called Windirstat that gives you a graphical view of EVERYTHING on your drive, how much space it takes, and more. Each of the colored blocks is data, when you hover the mouse over it, it tells you what it is. For example, the two big red blocks on the right are system files, pagefile.sys, and hiberfil.sys. Very cool!
Heres what it looks like: (I had to shrink it a lot to get it on here, but you can get more info at:

http://downloadessentials.com/current_software.php?software_id=56