What makes a great party game? It needs to be easy to pick up and understand, have a competitive edge and a skill ceiling that's not too high, and have games that don't last too long. Mix all that together and you've got a game you can play for minutes or hours, and anyone can get involved. In my experience, Golf With Your Friends fits this criteria perfectly, and everyone can play it for free for a limited time, with a massive update and Steam sale once you're done.
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One of Steam's best multiplayer games just got a big update
This 9/10 multiplayer game is free for the weekend, with a 67% sale