What are some alternatives?
When comparing KDirStat and WinDirStat, you can also consider the following products
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.
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.
TreeSize - TreeSize tells you where precious disk space has gone to.
Baobab - Distributor of workload automation, financial statement reporting, data access, IT Asset Management and Service Desk software solutions.
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.
DaisyDisk - DaisyDisk is a disk analyzer tool for OS X that visualizes hard disk usage and allows to free up hard disk space.