Windows only: A little sloppy file keeping can lead to a lot of duplicate and space-hogging files. Dig up all the dupes with speedy Fast Duplicate File Finder.
You can scan local, external, flash-based and network drives with Fast Duplicate File Finder. There are several convenient features like the ability to automatically mark the older of the two duplicate files as the one that will be deleted or moved, protection of system and program files, and a shortcut to immediately jump to the actual folder where a duplicate you'd like to examine is residing.
Fast Duplicate File Finder supports projects, essentially profiles, so you can customise your scanning technique and the locations to be included based on your various needs. There is in-program preview for images, video, music, text and binary files. When you find duplicates you can dump them into the recycle bin, delete them permanently, or have the files and their folder structure copied to a backup folder to serve as a holding area until you're certain you want them permanently deleted. Fast Duplicate File Finder is freeware, Windows only.