Digital Volcano's DuplicateCleaner
DuplicateCleaner not only allows you to search for files with the same MD5 hash value, it can also be used to find files with the same file name, file extension, date created, date modified, or file size.
The image mode of DuplicateCleaner can be used to find images of varying degrees of similarity.
If you know what duplicate files you want to find, there's an option under the File menu to import a list of duplicate files.
Thanks very much to Alfred Vachris for recommending this software!