Dungeons and Dragons

Good of you to ask. It's entirely possible to play D&D without buying any official materials (and you should play it without buying anything Hasbro currently sells, but that's just my opinion).

In fact, you can find a more or less complete set of rules for most versions of D&D just by searching combinations of the version you want and terms like "SRD" or "wiki". Some of these will lead you to officially hosted sources, and some not, but the great thing about D&D is that Hasbro can't ever sell it away from players.

I'm not going to provide any links to anything because someone will accuse me of breaking the rules, but D&D isn't Hasbro, and it wasn't even really TSR. It's just collections of rules, and game rules are not patentable. Hasbro owns a copyright in the 5e PHB's written content, for example (and some trademarks on trade dress and some terms), but crucially it does not own the way people play D&D. Ergo, in a matter of speaking, Dungeons and Dragons is already open source. If you've got a pen, some paper, and a fistful of dice, you can play. Less is more.

Having said that, many folks believe that the best versions of D&D aren't in print anymore anyway, but even if 5e is your version of choice (and to its credit, it has a few marks in its favor), I'd recommend checking a used book store before getting worried about whether this rumor ever amounts to anything. Hasbro can sell D&D, or not, and millions of people will happily keep right on playing D&D every week without ever giving them a dime.