I had a very similar problem and decided to compile various methods to solve it. So below are those options and some of them I got from the answers already provided here. I understand this is a bit off the topic of the question, but is in tune with the answers. This has many parts and those are all softwares that you could try in some way.
It is generally a good idea to pay close attention to the use of sweat options below so the software can access each file, which will probably include some big the hidden ones
Here is a short list of apps to check disk usage:
GrandPerspective is graphic only, using the Treemap, you can measure files by logical or physical methods before scanning, show / hide package contents and change color scheme on the fly. You can also save the scanned data to archive or compare multiple windows.
Disk Inventory X also uses the Treemap graphical schema, but in conjunction with a list view of folders and files. The graphics are not as good as GrandPerspective nor the list as good as OmniDiskSweeper, but it does a good job mixing the two. It has a Finder plugin and most of the options among the 3 preferences. It is the most complex, but not all complete.
OmniDiskSweeper is not graphical and is very similar to the Finder column view. If you choose the folder or disk to scan, it will sort them based on disk usage after taking your time to calculate. Then you can simply delete (trash) everything that appears in the list.
So each has its advantages and highlights, I'm still not sure if there is one that stands out. They are all free.
There's also a different approach, from apps to scanning specific places and expected files for space usage in less than optimal ways. Basically, they put together some familiar things about the system that may be bloating your disk into one nice interface so you can see and decide what to remove.
CleanMyMac lists caches, logs, language files, universal binaries, development "junk", extensions, and applications. It scans through files and also uses some knowledge base you have. Great interface, easy to use. CleanMyMac has a Free trial that will only clean up to 500MB.
XSlimmer is very specific. That remove "unnecessary" code from "fat" binaries Y Delete unnecessary languages, as it says on the website. In other words, universal binaries use a lot of space to store files that run on different architectures and languages. Therefore, this removes them all so that they are reduced only to the needs of your computer. XSlimmer is currently discontinued.
Another approach is to look for duplicate files. There are many trading options, some may be better than those listed below, I have not tried them all. Anyway, I'm listing my choice of apps considering which ones I could test.
TidyUp is a well-known application in this regard. You can specify where to explore what type of duplicates. It offers basic and advanced modes, several different strategies and criteria.
MrClean is a free tool that simply scans folders for duplicates and throws them away. Very simple but efficient if you are sure of what you are doing.
Chipmunk scans for duplicates and lets you choose which ones you want to throw away. It offers a folder node view and you can select "delete all files in a folder that have duplicates elsewhere, or vice versa"As well as manual selection. It can take a long time to scan all the files, but it does a very decent job after that.
DupeCheck "drop a file and it will use your Spotlight index to see if you have a possible duplicate somewhere"This is this good open source application. It is not a great space cleaning tool at the same time, but over time it helps you keep your space clean.
DuplicateFileSearcher from website: "is a powerful free software utility that will help you find and remove duplicate files on your computer. It can also be used to compute MD5 and SHA hashes. The software runs on Windows, Linux, Solaris and MacOS.". Enough talk.
Below, I will briefly discuss a similar approach by citing relevant parts about two other things that can be done to find missing disk space, without installing anything new, just using the command line (Terminal).
This (long but good) is from the MacFixIt forums (go there for more options and details):
In most cases, there really are files that take up part of the volume, but the files are invisible in normal Finder use.
Using the Go To Finder Folder function (on the Go menu), notice the content sizes of these folders by pasting these path names:
The / private / var / vm directory contains the swap files used by virtual memory. New ones are created as more data is exchanged from RAM to the hard drive. The entire creation process begins at every reboot or reboot; do not try to remove them yourself. Check the total size of all swap files, right after booting, and as the disk fills up. In Panther, the first two swap files are 64MB, then each new one is twice the size of the previous one (128MB, 256MB, 512MB, 1GB) up to a maximum size of 1GB. In Tiger, the first two swap files are 64MB, the next is 128MB, and the additional swap files are 256MB.
If you don't run the daily, weekly, and monthly maintenance scripts (either using a utility or running the sudo periodic daily, sudo periodic weekly, and sudo periodic monthly commands in Terminal), the records on the startup volume may become too large. If it fails frequently and is logging you may have a very large file in /private/var/log/system.log.
Files in / Volumes must be aliases for their mounted volumes. Do not remove these aliases, because everything you do happens to the contents of the corresponding volumes. If you are not sure that you can browse this folder smoothly, before you start, properly unmount any volume other than the boot volume, if the missing disk space issue only affects that volume. External FireWire drives can be disconnected after proper removal.
Sometimes, backup programs that cannot find a target (or target) volume intended for a backup create a folder with the same name as the target and place the folder in the / Volumes directory. There are cases where the entire startup volume has been backed up to a folder inside / Volumes. If the amount of missing space is approximately the size of your User Box, then such a backup is likely to be the explanation. If you use Carbon Copy Cloner or another backup or clone utility and have your preferences set to create a backup on a schedule, and the intended destination volume is not mounted or is inactive at the scheduled time, the backup it is created in the / Volumes directory.
To check the size of the normally invisible / Volumes directory on the active startup volume, open a new Finder window. Select the startup volume from the list on the left, then choose the column view (the one to the right of the three views). From the Finder's Go menu, choose Go To Folder and paste:
The / Volumes directory becomes visible in the Finder; Find its size by selecting it and typing Command I. My directory / Volumes is said to be 12K.
This one is from the Mac OS X Hints forums (not much else to see there):
You may want to run a du in the terminal to see what's going on. This may take a few minutes to execute.
An example would be to open terminal.app and then run these commands:
sudo du -h -d 1 -c /
Enter your password when prompted and then release it, it will take a few minutes to run, so be patient.
du means disk usage. There is also df. I like to include the
-x to the previous command:
sudo du -cxhd 1 /
By adding to the command line option, you can use an automation service to open any application. With this you will get different (and more complete) results in the GUI.
OR, if you are on a Power PC, using Rosetta or anything before Snow Leopardyou can mix any of the applications mentioned above with Pseudo. It is a small application to open things as administrator. Imagine it as a GUI for sudo.
Finally, there is a complete newbie guide to "The X Lab" that I am not going to quote here because it is too long.