How to clean out all "hidden" files on one's computer.

[Josef Lowder]

.
While I don't have any files or links on my computer that need to 
be hid from anyone's view, when I recently got the "popup" message 
shown below (one that Firefox failed to block), it made me wonder 
where all there might be files and/or "cookies" and/or other such 
stuff hidden on a Linux computer that might be good to clear out 
periodically. 

Here's the pop-up message I received: 

From: http://www.drivecleaner.com 

> Your computer tracked all of your actions and stored the evidence 
> on your hard drive.  Anyone could see where you had been on the 
> Internet, the images you had looked at, the sites you opened, the 
> movie you had viewed, etc.  We recommend to download DriveCleaner 
> to erase all evidence you have on your PC.

Other than clearing cache and trash, etc. where else are duplicate, 
redundant, and/or copies of other needless, space-wasting files stored 
on a Linux system that a "drivecleaner" program like this might remove?