I wrote this about, uh, forever years ago when I was really into .NET junk. It isn't impressive, but I still use it for some things.
You can search for files by name, size, extension, etc. It tells you what file and directory is largest, lets you delete files based on criteria (with or without confirmation), scans a tree and saves the data for later comparison (I wasn't very data structure savvy at the time so it isn't tremendously fast with large trees, but it is very thorough), and some other junk.
If interested, see it at my personal web site below:
http://kylej.name/page/windows-software.html#filesystem-tool