"Where did all my disk space all go?"
When I was in college, each student was allotted 4 megabytes ('mega' is not a typo) of disk space for both email and files. Granted, this was back in the early '90s, before anyone had heard of "www." or had even thought of ".mp3" or ".mp4". In 1995 I acquired a 1GB hard drive and felt like king of the known world. Sure, it was in a case about the size of a loaf of bread, but an entire gigabyte! All to myself! Today I have a Drobo which is in a case roughly the same size as that 1GB drive, but it has 2 terabytes of redundant storage -- 2,000x the space on that prime example of mid-90s storage tech.
One thing hasn't changed: there's still never enough hard drive space. It seems to be as unavoidable as death and taxes. That Drobo? It's about 95% full.
Enter OmniDiskSweeper, a freeware program by The Omni Group which will allow you to find where your diskspace has gone. Now, there are a number of different programs out there for analyzing your hard drive space: DiskInventoryX, GrandPerspective, Baseline, DaisyDisk, and WhatSize, to name just a few. Choosing one may be as personal as which Twitter client or web browser you prefer. I like OmniDiskSweeper for two reasons: it is free and it is simple. It uses the same "Column View" that I use in Finder, and automatically sorts the folders which use the most disk space to the top.
For example, looking at the Drobo, I can see that about 50% of the space on that device is taken up by programs I have recorded using Elgato's EyeTV. You might also find files in unexpected places. A friend recently used OmniDiskSweeper and realized that he had tried to copy a bunch of files to an external hard drive, but something had gone wrong. Instead of moving the files to the external drive, they had just been moved into a folder in /Volumes/.
OmniDiskSweeper makes it easy to delete files right through the app; simply select the file and click the big red "Delete" button. A word of warning! A warning dialogue box will appear, asking you to confirm that you want to "Destroy" the files. They will not be sent to the Trash (since the assumption is that you are running OmniDiskSweeper because you want to reclaim diskspace), they will be deleted immediately.
UPDATE: TUAW reader "ithinkergoimac" comments below that OmniDiskSweeper may report incorrect (significantly increased) file sizes if you are using TimeMachine.