Disk Space Fan 4 is a tool for quickly scanning drives or folders and viewing disk usage.
It is very easy to use, and has a colorful display that allows you to quickly view which folders or files are using disk space. Scans are reasonably fast, and the program remembers and can quickly show the last scan for each drive.
It also has a nice duplicate search function, which I imagine looks for duplicates right during the scan, as the feature ran in a second over a 100GB partition.
I recommend you try this program if you need to check you disk usage – it is fast, easy to use and has a 15-day trial available.