Before you run this make sure you play all your Steam games at least once or you’ll have to “verify integrity of game cache” of each which will force a download of the install files again. The UI is basic, you can click a box to select all games plus Desura and Steam install files so I went for the big one. I saved 17GB of space by deleting Desura game cache files I didn’t know I could delete. I found out afterwards that there’s an option in Desura to “Clean up MCF’s after use” too which is probably worth doing if you’re running short on space.
While we’re on the subject of saving space, download Space Sniffer to see where all your space is used.