TreeSize - TreeSize tells you where precious disk space has gone to.
Disk Inventory X - Disk Inventory X is an open source disk space analyzer and disk management tool for the Mac OS X operating systems that allow the OS X users to check for the availability of free space in their machines.
Baobab - Distributor of workload automation, financial statement reporting, data access, IT Asset Management and Service Desk software solutions.
TreeSize Professional - TreeSize Professional is a robust Graphical Manager developed by JAM Software to help you perform different operations on your storage systems like analyzing the data, organizing and freeing up the storage system effectively, and viewing the usage.
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.
1Tree - 1Tree is a useful application that makes it easy for you to navigate your hard drives and take various actions on folders and files like copying, deleting, or moving them.