How to find files that are eating up disk space

[John Almberg]

Here is another newbie question that is driving me crazy, but is  
probably a laughable situation to an experienced admin...

I've got a smallish server that is suddenly out of disk space in the  
'/' partition.

Probably some log files have gotten out of hand. I am going to start  
looking for the culprits by hand... basically inspecting sub  
directories, but there must be a better way!

Is there a command line tool that will help me figure out where the  
problem is?

Even better, is there a way to proactively monitor the file system,  
so I can fix problems before I start getting 'out of disk space' errors?

Any hints, much appreciated.

-- John