This game only works upto OS9, but it's my favorite mac game of all time, and deserves mention. Some of you may have played Netfungus, and this is the original with computer AI and everything!
It's very hard to find, as it was made by a kid in high school, and it's freeware, so I'll provide a link here.
http://archive.info-mac.org/_Game/_Board/fungus-20.hqx