Question: 1) What good software out there already exists that allows one (preferably visually) see what folders and files are taking the most space on one's computer (Windows Vista compatible)

2) Say I wanted to write a this program myself, what opensource tools out there can I use to get started?

Thanks!

Answer: 
1) You can try WinDirStat: <http://windirstat.info/>

2) I don't think you need open source tools, you could write it in Java or .NET just by using what is already provided with the languages.
Yes there are plenty of them. [For example check out these..](http://www.technize.com/5-free-tools-to-list-hard-drive-contents-graphically/)