Diskitude helps you figure out what is taking up space on your hard drive. Its intuitive and animated interface allows you to easily explore your file system and prune your files. Plus it's super small (only 10 KB) so it loads really fast and won't take up any space.
Actual size on disk is unfortunately a nebulous concept with NTFS (the format Windows uses to store data on disk), which supports compression, sparse files, alternate data streams, and copy-on-write snapshots. To calculate the actual size on disk I would have to write my own read-only NTFS driver. So I've chosen to correctly calculate what I can, which is the actual size of the file contents. File sizes all use binary units just like Windows where 1 GB = 1,073,741,824 bytes.
It is written in C using OpenGL and GDI, and has been tested on Windows XP, Windows Vista, Windows 7, and Windows 8. The executable compiles to 10 KB uncompressed because it uses raw Win32 calls instead of the C standard library. Working around the lack of basic math functions and full 64-bit integer support was a bit of a challenge, but it all worked out.
Update 8/21/2010: You can now press F5 to rescan, which uses an animation to morph the old scan into the new one so you can watch the files you deleted shrink and disappear. I also added Unicode support and a custom line renderer, and it still fits in 10 KB uncompressed.
Update 12/4/2013: Added a fix for sharp rendering at high DPI settings and managed to squeeze it back into 10 KB uncompressed.