Steam Cleaner is a portable tool for removing junk game files left behind by Steam, Origin, Uplay and GoG.
The program is as simple as this kind of application can be. It's a single executable, no installation required, no settings to think about, not even a "Scan" button-- just launch it and within a second or two you'll see a list of any game leftovers.
The developer said Steam Cleaner can clear up "gigabytes" of data in a few seconds. It found more than 100MB of junk on our tiny Steam setup, so this could be plausible.
If you're happy to wipe the files, click the Clean button, confirm you really, really want to do this, and the data is permanently deleted. (That is, gone forever, not sent to the Recycle Bin or anywhere else.)
Beware: like all third-party cleanup tools, Steam Cleaner may make mistakes occasionally and delete something important. For example, the release notes for 2.3 mention fixes for "Important Battle.net files being deleted" and "Fixed game files for Penumbra: Overture being removed".
The program caused no problems for us, freeing up 100MB plus without having any apparent effect on our Steam apps. But be careful, and back up anything important before you use Steam Cleaner for the first time.