What is filling up my hard drive so fast?
Here's the systematic way to find where the disk space went: Open a window in Terminal (you find this under Utilities), then type:

cd /
sudo du -sm *


This will give you the disk consumption, in megabytes, of every top level entry in the folder hierarchy. 

You could then drill down further, but without a good idea of what things do, you might easily decide on a wrong culprit (in particular, don't go around deleting random files in /System; that is bound to end in tears).

However, since your system keeps filling up by itself, you can just let it sit for an hour, and then run the du command above again. The difference between the two sets of numbers will show you where the space went.

In particular, if logging is a problem, you might see the number for /private go up.