Windows: Whether all of your games are Steam games, Uplay titles from Ubisoft or GoG purchases loaded into Galaxy, odds are after installing or uninstalling, you probably have duplicate files, leftover files or redistributables you don't need any more. Steam Cleaner takes care of them all for you.
Think of Steam Cleaner as CCleaner, but just for your games — and if you're like me, those games can take up a huge amount of space. That said, you also need to take as much care with it as you would with CCleaner, and make sure you're not randomly deleting everything is suggests. In our tests Steam Cleaner is pretty conservative with what it will scrub out, and only targets things like redistributables that you already have or are shared between games, or files left behind after you've uninstalled a title that Steam doesn't delete for one reason or another. You do, however, have to make sure those left behind files aren't save games or anything.
Steam Cleaner has been around for a while, in both installable and portable versions, completely free and Windows-only. Give it a try and see how much space it clears up for you.