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.
WizTree - WizTree quickly finds the files and folders using the most space on your hard drive. It scans the MFT (Master File Table) instead of crawling the entire disk which makes it very fast.
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.