Blizzard, having apparently decided that there are not yet enough people playing Overwatch, is making the game free for everyone over the weekend of September 22-25. The freebie will include the full roster of 25 heroes, 16 maps, and access to modes including Quick Play, Custom Games, and the Arcade, and any progress earned will carry over to the full game if you decide to pick it up.
The free period will begin at 11 am PT/2 pm ET on September 22, and wrap up at 11:59 pm PT on September 25. If you don't already have it, you'll need Battle.net to play, which you can grab here. Alas, it will only be playable in the Americas, Europe, and Asia—excluding Korea, for some reason—so if you live in Antarctica I guess you're out of luck. Details on the Overwatch free weekend (although really, what more to do you need to know?) are up at playoverwatch.com.