[redhot_nyc]

I have an iMac with 20GB HD, OS 10.2.8

Applications, Library, System & Users directories total 4GB

I have 400MB HD available... so where did the extra 15GB go?

[ryaxnb]

Three tips:
1. The Mac keeps a lot of files hidden, but I don't think it keeps nearly 3.7GB hidden. It might, though, if you've installed a bunch of UNIX programs or something.
2. There could be stuff in your Trash. To empty the Trash, switch to the Finder and choose Finder > Empty Trash.
3. If you're running a multi-user system, there could be stuff in someone else's Trash. If you have administrative rights (you might as well try this out - it will tell you if you don't,) use these UNIX commands (at the Terminal, which is located at Macintosh HD:Applications:Utilities, or /Applications/Utilities in Go To Folder):
cd /users [hit the enter key; in the future, I'll write [Enter]]
ls [Enter]
[This is not a command] Decide which user account to peek in
sudo cd [user account folder goes here]/.Trash[enter]
Enter your password when asked. If you have no password, leave the field blank.
sudo rm -rf *[enter]
Repeat, if necessary.

[yukon]

yeah, that last one will clear a lot of space....maybe a little too much.....depending on your shell.

[bmhome1]

Just download the trial version of OmniDisk Sweeper. It will display in descending order every single file in your OSX install. It will graphically show what files are taking up so much space. Probably swap files.