Yes.
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Assuming you're referring to Defense of the Ancients, it's free, but the game that plays it is not.Basically, you have to have Warcraft 3, but DOTA itself is free, you just can play it.
Steam
There are a number of places where someone can access a free DotA map. Some website examples include Get DotA, DotA-Utilities, Softpedia and DotA Source.
Defense of the Ancients (DotA) can be played with Mac OS X.
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