Halo RTS spin-off Halo Wars is having a free weekend on Steam that will start next Thursday and run until Sunday. It'll be worth jumping on: it's a solid RTS with a decent campaign that you'll enjoy even if you're not a fan of the Halo series. A weekend should give you more than enough time to get a feel for it and decide whether to buy it—or, you could just blast through the story in a couple of days and then leave it alone if you're not fussed about the multiplayer.
It's the Definitive Edition (opens in new tab), which arrived on Steam last year (opens in new tab), that will be free from October 18-21, according to Gamespot (opens in new tab). Halo Wars was first released on the Xbox 360 in 2009, and came to the PC remastered in 2016, but only bundled with its sequel, Halo Wars 2. The Definitive Edition is the standalone version.
There's no word yet on whether the game will go on sale during or after the free weekend: it's currently priced at $19.99 on Steam.