Not sure if I'm late to the party and everyone already knows about this but I found an article on PC Gamer that mentioned a program called Steam Cleaner. Basically it goes in and deletes old files from when you download games that may linger. Its a small download and it search my hard drives in a matter of seconds. It cleared up 5 gigs for me and 2 gigs for Voldenae. Check if out if you never heard about it.
He's not lying about the "matter of seconds" thing either. It was so quick I was sure I'd done it wrong.
That got rid of 9088 files. I have no idea how much space that was. I think it looked at my battle.net stuff as well.
When I ran it months back, it removed some 10 GB of files.
From what I hear though, Valve is changing the way Steam caches common install files so this utility won't be needed any more.