Kompressor

^ Basically it's a low-res, mostly opensource open world platform that follows MMO economy rules: Do something, make something, build something. It's popular because it is easy to create a homebrew server from an old household computer that you and your friends can play on.

Unlike other MMO's however, the only limit to the world you make is your own imagination and perseverance.

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Think like a man of action, act like a man of thought. -H. Bergson