The competitive multiplayer aspect of Frontier Development's space title Elite Dangerous, Arena, is currently available free of charge.
This is true on both Steam and Frontier's own store. The title will be free until Monday, July 11th. Normally this will set you back 4.99.
Note that this is the standalone version of Arena – if you already own either Elite Dangerous or Elite Dangerous Horizons, you will already have this content.
Elite Dangerous was famously crowdfunded on Kickstarter before launching on PC in December 2014.
Since the title has come to Xbox One with a PlayStation 4 version incoming at some point in the future.
The title was an Oculus Rift launch title when the hardware was released earlier this year.