Even though they may not have the glitzy 3D graphics of bigger name games, I'd suggest checking out the shareware games offered by Ambrosia Software. They have great playability, low system requirements, and they're just plain fun. One of my personal favorites is the Escape Velocity series, only one of which will run on OS X, Escape Velocity Nova, which is loads of fun
Visiting sites like www.download.com and macupdate.com list hundreds of shareware or freeware games. Many of them are a lot of fun. I would recommend sorting by user rating.
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