Good recommendations so far.
Here is one I just started using.
Duplicate Cleaner
My music and video collection was a mess, files everywhere from different users on the PC. Plus I had two hard drive crashes without a good back up (I had just copied one time from my PC to a Mac over the network). I found a program that allowed me to save most, if not all my media files, (should recommend that too, but I can't remember the name) but the file names were lost. The duplicate cleaner found all the files that were duplicates and allowed me to free up a lot of space. I did the same thing to my Android phone.
A couple others I've used a lot are CDex and CutePDF (free print to PDF file creator)