iStat Menus - "An advanced Mac system monitor for your menubar."
WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
Process Explorer - The top window always shows a list of the currently active processes, including the names of their owning accounts, whereas the information displayed in the bottom window depends on the mode that Process Explorer is in: if it is in handle mode you'l…
TreeSize - TreeSize tells you where precious disk space has gone to.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
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.