My harddrive is full, but according to the size of all my folders, I should have more than half of my drive left over.
Short Answer: WinDirStat
NOTE: When you run it, make sure to "Run as Administrator".
Long Answer: Unfortunately Windows Explorer (File Explorer) was dead wrong about the size of my “Program Files” folder. It said that my “Program Files” folder was 16.5 GB. However, inside of it I had an .LDF file that was over 200 GB! Within about five minutes of starting WinDirStat I could see what was hogging up my drive space.