WinDiff is a useful application (in MS Visual Studio) to compare two files or folders, WinDiff UI is a better front end to WinDiff. It is a freeware file comparison application that's easy to setup, light on system resources, easy-to-use, vital, user friendly and straightforward.
Here are some screenshots to illustrate the features and user interface of this application:
Bulk Rename Utility
DupScout
Araxis Merge
TreeSize Free
Directory List & Print Pro