Such is Rocket League's ubiquity nowadays that it's hard to imagine a world without it. Vehicular future soccer is an unambiguously excellent thing after all, a necessity we didn't know we needed until it released. Still, there are probably many out there that haven't played it yet, but they'll have no excuse this week.
Rocket League is currently free to play on Steam, and will be until Sunday morning (I assume this is PT time – that's roughly early Monday morning in Australia). Plus, if you're a newcomer and like what you play, the game is also 30% off until that time. That applies to the GOTY edition as well as "most" DLC.
As for that DLC, there's a tonne of it now, most recently in the form of a Fast and the Furious crossover. Which suggests to me that maybe, some day, Rocket League will get its own film adaptation. We can only hope.
