This weekend, Sony is hosting a multiplayer open weekend for PlayStation 4, letting anyone - even those without a PlayStation Plus membership - play games online.
On PlayStation 4, a PlayStation Plus membership is normally required to play the online multiplayer components of games (with some exceptions, like subscription-based titles.)
However, from 7 pm on Friday, September 26 (tomorrow) until 7 pm on Monday, September 29, that requirement is being lifted, allowing anyone with a PlayStation 4 to get a taste of online gaming.
This free weekend is obviously designed as a way to entice people to subscribe to PlayStation Plus, but it gives players a chance to try out the online multiplayer components of games like Diablo III, The Last of Us Remastered, FIFA 15, and Destiny (which is an always-online game, but requires PlayStation Plus for certain, group-based activities.)
Source: PlayStation Blog