These days, there are tons of free utilities out there to help improve your system performance in some way. What I have found is that some of them overreach, and many of them over promise and under deliver. That's why I like my utilities solid and customizable.
Easy Duplicate Finder does just what it says. It finds duplicate files. There are a lot of utilities out there that do that, but the reason this one is on top is because it does exactly what it is supposed to do and nothing more. Even better, it has plenty of easy to use options that make sure the results you get are the results you want.
For example, take my computer. I have 50GB of photos stored on my hard drive. That's right, FIFTY GIG! Why? Well, I do shoot my pictures in RAW mode and I do have an amazing two year old, but that isn't the big problem. Before I learned how to actually use good digital photography software properly, I did a bunch of things manually with my pictures. For example, I would make a copy of my pictures as soon as they were loaded onto my disk so that when I edited a picture, I didn't accidentally write over the original. Then, I made other copies so that I had all the pictures I wanted to order online in the same directory for easy uploading. I made another directory so those files could be shared and so on and so on. In other words, I have duplicate photos all over my hard drives (I have two). The catch is that I can't delete any of them because I don't know which ones are copies, which ones have been edited, and if I deleted the copies that I think there are the last time I undertook a project like this.