WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
DocFetcher - DocFetcher is a portable German/English open source desktop search application.
TreeSize - TreeSize tells you where precious disk space has gone to.
Everything by Voidtools - Everything. Locate files and folders by name instantly. Everything. Small installation file. Clean and simple user interface.
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.
Agent Ransack - Agent Ransack is a tool for finding files and information on your hard drive fast and efficiently.