Hard Drive Space Diagnostic?

Is there a third party program out there (or even a built in OSX program I don't know about) that can tell me where I am using most of my hard drive space?

I'm nearly using all of 80GB but i don't know where to find it all, and I want to off load most of it to my new Lacie 500gb.

Any help is GREATLY appreciated!

thanks for reading,
-Uli

Grand Perspective works well.

Disk Inventory X also works rather well.
http://www.derlien.com/

As does What Size.
http://www.id-design.com/software/whatsize/

Thanks to both of you! I tried "WhatSize" and it was perfect. Although, I did have to pay 12$...but it's reasonable considering I rescued 20.58GB of wasted space.

If anyone seems to be MISSING hard disk space and you are using an external hard drive you might want a program like "WhatSize". The program will show you how much space each individual folder/file is using. You just select a folder (e.g. Hard Drive) and it will list every folder within in order of size. Very easy to use.

What happened to me was this:
I was connected wirelessly to my Lacie (500GB) through my Airport Extreme, at some point I lost connection with the Lacie...no big deal, BUT at the time of disconnection I was downloading a huge file. My computer kept downloading the file, but it was no longer sending it to the Lacie, and so it was saving the downloading file to some phantom folder. This happened about 10 times before I realized it, hence the missing 20+GB.

Hope this helps someone,
-Uli

I'm on a PowerBook G4 with OS 10.4.11