How can I find out what is taking up space on my hard drive?
I highly recommend using:  http://daisydiskapp.com 

.. Once downloaded and installed they have a "Test Drive" option you click that lets you use it for free.  Please note that to see all files on your HD that all users own, you will want to use the "Scan as Administrator" option, which will prompt you to enter your password.

There is also:


Disk Inventory X - http://www.derlien.com
Disk Map - Google for "Disk Map OS X" ( Commercial software )