I've been a long time user of SpaceMonger and i thought i'd point you to it.  I find it to be incredibly useful for figuring out where your space is being used quickly and intuitively.  For example, i often use VPCs and they have *huge* disk images.  Every now and then i toss one into a folder somewhere and then totally forget that i did that.  Then, later i realize "holy crap! i'm out of space!".   I can just run this tool and immediately see the problem at hand.  Hovering over a region will tell me the total size it occupies, and by double clicking on any colored region, i'll then get a zoomed in view of that folder with a whole new level of detail.  Very very useful.  (I also think the display is very pretty (except for the buttons at the top) and simple to use and understand).

Here's a view of my disk right now.  You can see a couple of interesting things in it.  One is my private branch (csharp_private_cyrusn) where i do all my own source code management and hammer out my stuff on my own.  Then, there's the private branch that my team uses (lab22dev) where my code will be integrated into once it meets a certain code quality level.  "lab22dev" is actually a misnomer and should be called csharp_private.  csharp_private is actually a branch off of lab22dev, which is a branch off of lab22, which is a branch off of "main" where the official builds come from.

In this case the tool let me discover that i was using 1 GB in my Desktop\BadThings directory.  This is a location i keep around for things that make me sad.  In this case i had a crash dump in our code that i had investigated and fixed, but hadn't remembered to delete.  So while running this tool normally makes me happy because it's so pretty, in this case it made me sad because it reminded me of bugs we'd had in the past.