WinDirStat is a port of KDirStat for Linux. It's lightweight, small (650kb installer), fast, portable (as a standalone .exe file), and works on multiple versions of Windows. Besides showing folders and percentages (for the entire disk or any subset of folders), it also displays an (optional) graphical usage map. Works well with NTFS Junction folders, avoiding counting folders multiple times.
SpaceSniffer is another possibility. It can scan Alternate Data Streams (ADS) and correctly ignores junctions. However, it is not hard-link aware. If a file has multiple links, they will show up in the scan more than once. I've personally tested all this information to be accurate on Windows XP as of version 1.1.2.0.
FolderSize, since it's integrated into Explorer and caches the results. You always know how much space a folder is taking, and can easily identify space hogs. It's always there, so you don't have to start a separate program.
TreeSize Professional is a powerful and flexible hard disk space manager for Windows 8/7/Vista/XP or Windows Server 2012/2008/2003 (32 or 64 Bit). Find out which folders are the largest on your drives and recover precious disk space. Use TreeSize as a hard disk cleanup tool: find space hogs and remove them. Graphical analyses provide a quick visualization of disk space usage. TreeSize Professional shows you folder size and allocated disk space as well as owner and permissions, the last access date, the NTFS compression rate, and much more information for selected folders or drives.
JDiskReport enables you to understand how much space the files and directories consume on your disk drives, and it helps you find obsolete files and folders.The tool .analyses your disk drives and collects several statistics which you can view as overview charts and details tables.