I’ve never really engaged in all that many free RPGs in all honestly. From what I know several are very very good, such as Stars Without Number and others are very very weird such as Wisher, Theurge, Fatalist & Weaver of their Fates. The latter being so very abstract that it needs to be read to be believed (but probably not understood).
Honestly though, it’s probably Eclipse Phase. Not only is this an amazing setting that channels the greatness of Richard Morgan’s Takeshi Kovacs novels, but it’s also a high end book with amazing production values. Now the book isn’t free but the pdfs are, released under a creative commons licence and this alone is a bold and brilliant move by Posthuman Studios.
Eclipse Phase is so good that, even with all the PDF’s being free, I’d buy the dead tree version of the game if I thought I had any chance of running it any time soon. It’s a game that I recommend everyone to check out.