Not sure if it can really be considered hidden, but many people don't seem to know about TreeSize. It's an incredibly useful tool to find out what takes up space on your drives.
Unfortunately, WinDirStat is essentially abandoned... The latest version 1.1.2 was released in 2005. Wiztree is my favourite alternative, but it's not open-source.
There's an easy solution to that. If you donate, it'll remove the button.
It's a good deal. The money saved from not having to buy new drives / not having to manually calculate usage yourself would be far greater than any donation.