VisiPics - VisiPics does more than just look for identical files, it goes beyond checksums to look for similar...
WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
dupeGuru - dupeGuru is a tool for finding duplicate files on your computer.
TreeSize - TreeSize tells you where precious disk space has gone to.
Pixiple - Find images similar in pixel and/or metadata content
SpaceSniffer - SpaceSniffer is a freeWare (donations are welcome) and portable tool application that lets you understand how folders and files are structured on your disks.