Czkawka is a simple, fast, written in Rust and easy to use app to remove unnecessary files from your computer.
Duplicate Cleaner is a tool for finding and removing duplicate files from your computer or network drives; It is intended to be used on user content - documents, photos, images, music, but can be used to scan any type of files.
AntiDupl.NET can find and display duplicate images in the main graphic formats: JPEG, GIF, TIFF, BMP and PNG. The comparison is based on the contents of the files, so the program can find not only almost identical, but similar images.
VisiPics does more than just look for identical files, it goes beyond checksums to look for similar pictures and does it all with a simple user interface. First, you select the root folder or folders to find and catalogue all of your pictures.Discontinued
The program seems to be no longer updated. Last version, 1.31, released in February 2013, can be still downloaded from the official website.
FSlint is a utility to find and clean various forms of lint on a filesystem. I.E. unwanted or problematic cruft in your files or file names. For example, one form of lint it finds is duplicate files. It has both GUI and command line modes.
Pixiple searches your files for images that are similar in pixel and/or metadata content and presents you with a sorted list of similar image pairs to compare and manipulate. Pixiple is open source, no-nonsense, unrestricted and completely free.
Awesome Duplicate Photo Finder is a free powerful tool, that helps you to find and remove duplicate photos on your PC. With this program you can easily clean up your photo collection from duplicates or even similar images.
SearchMyFiles is a non-indexing portable alternative to the standard "Search For Files And Folders" module of Windows. It allows you to easily search files in your system by wildcard, by last modified/created/last accessed time, by file attributes, by file content...
Anti-Twin is a small software application which compares files, i.e. it searches for duplicate or similar files on your hard disk drive. All similar or identical files that were found can either be sent to the recycle bin or directly deleted.Discontinued
Last version 1.8d from 2010, see http://www.joerg-rosenthal.com/en/antitwin/