Steam Surgeon, is what this would be called, if I’d made it. But because I’m more likely to make something like this and only very sensible people make useful apps for cleaning up your games library, it’s called Steam Cleaner instead.
What Steam Cleaner does is delete any redistributable packages found among the files of your Steam games - the innumerable .net and DirectX installers you’ve accumulated over the years. You can download it here.
Our Tim has reacquainted himself with about 10GB as a result - bear in mind, though, that our Tim has all the games.
So: what do you plan to do with all that new floor space?