WinMerge

This tool is a must-have for all system administrators and developers. Apart from the search for duplicates, this soft can also compare folders and merge different text files. Now this open-source tool works for Windows only. However, the next version will be compatible with Linux, too. WinMerge has a very useful feature: it is possible to run the tool from the executable file, so there is no need to install it as you can upload it to USB drive and run it from there.