XCOM 2 is now available for pre-purchase, if it's something you're planning to throw your money at up-front. To mark the big moment (and also the real reason we're here), 2K Games has made XCOM: Enemy Unknown free for the weekend on Steam—and the weekend, at least on the 2K bizarro calendar, has already begun.
Pre-purchases of XCOM 2 will include the Resistance Warrior Pack, which enables customization of your resistance fighters with new outfits, headgear, and war paint. It will also unlock a "survivor of the old war" as a recruitable character, "instantly," according to the Steam description, implying that you'll be able to unlock him or her through other, more laborious means (or maybe just pay for it) as well.
As for the "original" XCOM: Enemy Unknown ("original" in quotes because, as you'll recall, Enemy Unknown is a remake of the brilliant 1994 release X-COM: UFO Defense), it will remain free until 10 am Pacific on September 13, and will be on sale for 75 percent off its regular $30 price during that period as well. Progress earned during the free weekend will carry over if it's purchased—although if you're anything like me, three days of play will be just enough time to figure out what an absolute mess you've made of things, and that starting over is the only option left.