Windows only - Read below for CLI on *Nix
Lets face it - We're all guilty of having tons of DVD's and ISO's - But sometimes it can be dumps, log files or misc files that take up a lot of space.
Simply launch the app (Run as admin so it can scan your entire drive), wait whilst it goes through your drive and will display a nice graph of where your space is going.
Screenshot - Credit to TreeSize
Linux - I recently discovered this and fell in love with it immediately.
I kid you not, it scanned a 4TB SSD/HDD (RAID) in < 5 seconds on first install; after running it multiple times, it got down to 2 seconds...Yes, two seconds.
Lets face it - We've all experienced the common problem, you uninstall a program, and you can't delete it's files/folders.
Or maybe you've closed a program, try to delete the file/folder, and it says you can't delete the file/folder because it's already open in another process.
I used to use a program called Unlocker, however for whatever reason, it stoppped working for me (Win7 x64 as admin).
I hope this solves your frustrations of pilfering through task manager, rebooting your system etc.