Windows: If you're looking for a snappy and portable disk scanner to dig into your disk usage in an easy to digest radial-chart format you'll want to check out Scanner.
Scanner is lightweight, portable, free, and it makes short work of scanning multiple disks. Scanner displays the results as a radial graph. Each slice of the pie is interactive and drills down deeper into the folder structure of your PC. Radial graphs are great for visualising disk usage and quickly targeting which folder and sub-folders are doing the majority of the space hogging.
For another — though not as compact — disk scanner with a radial interface, check out previously reviewed OverDisk.