Non-Linux FOSS: File Spelunking with WinDirStat
With Linux, it's fairy easy to find the large files on your system by doing something like this:
du -ahx / | sort -rh | head -20
Unfortunately, Windows users don't usually have equivalent tools. That's where something like WinDirStat comes into play. It's a file browser that uses incredible GUI elements to show you the files on your system with file size shown as rectangles. Big files are shown as big rectangles, and their file types are specified by color. It's a great visual way to sort your filesystem and get rid of (or at least find) extremely large files.
(Image via https://windirstat.net
If you use Windows on a regular basis, but seem to have a shrinking hard drive, I urge you to download WinDirStat to get real-time statistics on your filesystem. It's open source and, of course, free to download here.