It really depends on what you're looking to acomplish. If you want to compress your drive to fit more on it, like many of us did in the 80's and 90's with software like doubledisk, stacker, etc, my first bit of advice would be to *stay away from that junk*
That said, keep in mind that much of the stuff that fills our hard drives these days (videos, music, images) are already in compressed formats like divx, xvid, quicktime, mp3, aac, jpeg, png, so compressing them again isnt going to yield spectacular results. in some cases it can even make the file larger.
To be honest you're best bet is to simply mow some lawns and buy a bigger hard drive.
Hope that was helpful.
Cheers,
Darren