Elite Dangerous: Arena, the stand-alone Close Quarter Combat mode of the space trucking sim Elite Dangerous, is currently free for the taking and will remain so until 1 pm ET on July 11. And if you grab it at any time during the freebie weekend, you can keep it forever.
To be perfectly clear, this is not the full Elite Dangerous, but rather just the PvP dogfighting bit—which I'm sure a lot of people would say is the best part anyway. And there is a connection between the two, as they're cross-play compatible, and credits earned in Arena combat can be used in the full game.
Do be aware that Arena is part of the full Elite Dangerous game, so if you own that, you already have access to Arena and probably shouldn't re-buy it. That's not really an issue as long as it's free, but something to keep in mind when the price goes back up. Elite Dangerous: Arena is available from Steam or the Frontier Store.