Mac hard drive almost full; diagnostic apps do not explain why 157gb of my 159gb mac is showing as used.  In the storage area 145gb of this is showing as other.  What can this be? 
I have just downloaded whatsize app to understand and whilst the 157gb is showing as used on hard drive when you click into the detail within whatsize it does not add up to anywhere near this.  About 20gb is broken down in the detail.  Would really appreciate some help as I'm at a loss I have not been able to use my mac properly for months now and when I took it to a computer repair shop they tried an omnisweeper but this made no difference.
Is this a MacBook? I ran into something similar and it turned out to be the local Time Machine snapshots. This was a new feature in OS X 10.7 Lion. Some programs show that space as used and some don't. Finder doesn't show that space as used but Disk Utility does.

Make sure to run WhatSize in "As Admin" mode to get an accurate representation of your disk usage. This isn't available in the App Store version. In that mode it will show you the local Time Machine snapshots.

Here is more information about local snapshots.