This one is tricky. I don’t tend to get free RPGs, often, or when I do, it’s usually a sale on a RPG Drive Thru. Most likely, I’d have to go with Atomic Highway by the great Colin Chapman, or Lords of Olympus, which I had gotten free, I don’t remember why. Most likely a sale.
Of the two, I find the latter more useful for the massive campaign setting information on the various gods, major, minor, and the Titans. It’s a very accurate guide to Greek myth, or as accurate as you can be with fiction. I’ve used it for the Battlestar Galactica game, and occasionally for guiding my inquiries for research Hercules. (It’s like mining a secondary source to find the primary ones, or using Wikipedia to look for better sources…)