Update: Total War: Shogun 2 is shedding its price on Steam today, letting armchair shoguns lead their armies across Japan for free. The offer starts today at 6 pm BST/10 am PT and will conclude at the same time on May 1. If you snatch it up between then, you can keep it forever.
Sega's also hosting a Total War sale, starting at the same time. Select historical Total Wars will be discounted, though this lamentably doesn't include the Three Kingdoms or Rome main games or DLC, and obviously the fantastical Warhammer's won't be part of it.
Original story: SteamDB keeps a list of upcoming free promotions so you can keep abreast of all the free weekends on Steam in advance. It also notes when games will be free-to-keep, which is why we can see that Total War: Shogun 2 will be given away to anyone who grabs it between April 27 and May 1.
Shogun 2 is nine years old but it's still my favorite of the historical Total War games, with a perfect map size to feel conquerable without having to throw away hundreds of hours and a theme that breathed life into blocks of troops marching across maps. Just last year Malindy Hetfeld reminisced about how beautiful and ambitious it was.
If you want something free right now, here's a list of what's currently being given away for zero dollars. This week Ubisoft will be giving away Assassin's Creed 2 as well, though that will be through Uplay rather than Steam.