WinDirStat is a disk usage statistics viewer and cleanup tool. It allows you to easily see the files and directories that make up a drive and determine what's taking up space. You can even remove files from within the interface. More details are on the WinDirStat homepage.
Download WinDirStat Portable 1.1.2 Pre-Release 2
(may take a few minutes to be available)
Let me know what you think.
UPDATE PRE-RELEASE 2 (August 23): The ANSI version (Windows 95/98/Me) is now included as well as the Unicode version (adding only 250k). The launcher will automatically choose the appropriate one based on what OS you're currently on.