While looking for ways to free up space on my Macbook, I found a free tool called Disk Inventory X which analyzes your files and spits out a neat visualization:
Hovering over the large green rectangle, for example, shows me that the 15 GB IE8 VDI image I downloaded today is taking up a lot of space. Colors are used to mark similar file types so the maroon blocks, for example, are MP3s.
Preceden uses a similar combination of size, color, and mouse-over details to visualize time. It’s amazing how much you can do with just those three things.
You can download Disk Inventory X for free from its website here.
Matt Mazur 
PC users can download WinDirStat, which is basically the same thing. The Disk Inventory X page says the author got the idea from WinDirStat.
Looks like it originally came from the Linux utility KDirStat. I guess every OS has a version now!