Warhammer 40K: Rites of War will be free on GOG until 6 am PT/9 am ET on July 19, while The Warhammer Weekend sale runs until the same time on July 20.
This turn-based strategy game is available
on this page
where it's yours to keep if you pick it up over the next two days. Other games
in the sci-fi series are up to 75% off, and you can find them listed
right here.