WinDirStat is a program I've used before to analyze what files are filling up a hard drive and it works well. The graphical portion of it seems pretty gimmicky but actually ends up adding some usability to the whole thing(if you're looking for a particular file extension, you can click that on the right hand side and then look for large highlighted boxes at the bottom.)
I haven't tested it in Windows 8, though, but I'd be surprised if it doesn't work.