So... I buy a mac partly so I won't play games...

And then I found Enigma:

http://mac.softpedia.com/get/Games/Enigma.shtml

And now I've gone and found Freeciv:

http://freeciv.wikia.com/wiki/Main_Page

I don't play online (out of some sort of compassion for respecting my free internet access for work)... But playing against the AI is pretty fun.

You can play freeciv on Windows too (if you really really want to).

Freeware... Unbelievable.