Where did my hard drive go?

I've said before that mobile hard drive capacity seems to be the only thing that doesn't keep up with Moore's Law, but this is getting ridiculous.

It seems like space on my MacBook Pro hard drive is disappearing. In 1992 I had a PowerBook 100 that came with a 20MB hard drive (that's megabytes, not gigabytes) and thought that I'd never be able to fill it up. My PowerBook 540c had a 320MB HDD and my 5300CE/117 had a 1.1GB HDD. Again, all of these drives kept up with my storage needs quite nicely.
Today's MacBook Pro ships with a 100GB HDD standard and for an extra US$100 you can upgrade it to 120GB, but if you want the faster 7200RPM drive, that tops out at 100GB. My problem is that I tend to be a data pack rat and don't archive as much as a I probably should. My PowerBook G4's 100GB HDD filled up fast and now my 120GB is starting to swell. I just can't seem to stick to the golden rule of maintaining 20 percent free space for "optimal system performance." Whatever that means.
If you've also been victimized by a vanishing notebook hard drive, it's time to take a long hard look at where all the space is going. For starters, if you don't use GarageBand, you can easily lose 1.5GB of Instrument files that are in
    HD > Library > Application Support > GarageBand > Instrument Library
    
If you don't use iDVD, there are about 1.7GB worth of themes you can toss. Everyone has a different strategy in archiving their music and photos but my rule of thumb is that both of them combined shouldn't be more than half of your HDD capacity. It sounds ridiculous I know, but some people never archive their music and photos. It doesn't help that Apple makes archiving and moving your iTunes as difficult as humanly possible.
An absolutely indispensable application for helping you recover lost hard drive space is Tjark Derlien's Disk Inventory X. Disk Inventory X is a free disk usage utility for Mac OS X 10.3 (and later that displays the sizes of files and folders in cool graphical "treemaps" (pictured). When you hover over an especially large blob it tells you what the offending file is. They're also color coded so you can quickly tell what types of files are taking up the most space.
The author got the idea for Disk Inventory X after seeing a friend's version for Windows called WinDirStat, short for Windows Directory Statistics.
Run, don't walk, to download this sweet little application and make some space so that you finally have some breathing room and maybe even install Parallels or Boot Camp.