I used a program on my older computers to visualize the disk space used and the various file sizes. I don't recall the name, but it would split the window into rectangles with a size proportional to the file or directory size. Very handy for finding those large files and directories that are hogging the disk space. When doing simulations of FPGAs the data is saved on the disk and you have to remove the files manually. They can get very large, sometimes GBs each.
I couldn't find the program I used to use, but I found one that works even better. SpaceSniffer. It draws simple boxes without any special effects like shading and seems to run quickly.
No Linux version it seems.
One thing that works better in this program than the old one I used is the ability to specify a starting folder rather than having to scan the whole disk drive. That can save a lot of time if you just want to look at one area.
WinDirStat is also mentioned a lot in the reviews, but the window is done with shaded boxes that look like they have a light shining off them. Not sure I'd like that.