Battle for Wesnoth is one of my favorite Mac games of all time, and best of all, it's completely and totally free. It's an open source game back when creating open source games was the thing to do, but unlike most other open source games, it has endured and flourished, currently sitting at version 1.11.0. Battle for Wesnoth is a turn-based strategy game, and it's really excellent -- it's based on a filled-out fantasy world, there are plenty of units to fight with and against, there's a full story mode to play, and there's a skirmish mode where you can just pit different factions against each other to your heart's content.
And if that doesn't give you enough to do, there's also a ton of great user-made content campaigns, which bring all-new stories and content to the title. And since it's all open source, you can even jump in and create your own art, scenarios, units, or maps for the game. There's an unbelievable amount to do with this engine, and again, it's all for free.
Battle for Wesnoth is a classic game (that's also available for iOS, by the way, thanks to a hardworking developer), and as far as I'm concerned, it's a must-install on every Mac I've ever owned. Definitely go grab it and play it -- it's available for download for free from the main website.