WinDirStat
I like WinDirStat, a free (open source) tool that scans the file system and visually presents statistics about your files. You can use this to determine which files are the largest, where is everything located, etc.
alt text http://windirstat.info/images/windirstat-small.jpg
From the site:
WinDirStat reads the whole directory tree once and then presents it in three useful views:
- The directory list, which resembles the tree view of the Windows Explorer but is sorted by file/subtree size,
- The treemap, which shows the whole contents of the directory tree straight away,
- The extension list, which serves as a legend and shows statistics about the file types.