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Honestly, I don't know any free games that are good at all.

Oh me oh my, there are many good free games. There are entire communities devoted to them, some of the best of the top of my head;
Cave Story, Mr. Stumps Dentures, Alex Adventure, The Spirit Engine, Eternal Daughter, Knytt and Knytt Stories. And if I'm being bigheaded The New Satan Sam :p

Don't know about free games for Mac though but all them games aren't high powered so would easily run on Parallels or whatever the younguns are using these days.
 
Ambrosia's Escape Velocity series are shareware rather then free but are excellent, huge games and well worth trying out.
 
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