Windows: If you're on the hunt for which folder and files are chewing up the most hard-drive space, Disktective is a free and portable app that makes short work of it.
Disktective scans any drive or folder you point it at and returns a list of the folder and file sizes contained therein, and a pie chart displaying the portion of the disk or folder in use by the folder or files in question. Once you scan, you can drill down through to sub-folders and the chart will automatically update to reflection the ration in the new folder.
If you want to visualise your data as a radial map, check out previously reviewed OverDisk. Diskective is free, portable and Windows only.