The link above points to a brief review of the DiskState tool that I wrote up a while back. I wrote the article in response to a request from someone who was looking for a tool that could find duplicate files on their hard-drive. Finding duplicate files can save you a lot of disk space if you have multiple copies of your music and/or picture files on your hard drive.
There are, naturally, many other tools that can help you to find duplicate files on your computer. Programs like google's free Picasa 2 and Microsoft Office's Picture Manager are able to find duplicate photos for you, and there are countless little utilities devoted to finding duplicate mp3 and wma files on your computer.