It's not new or anything, but if you want to experience what made Bungie and the Halo franchise into what it is today, you can download the marathon trilogy from Bungie's site:
http://source.bungie.org/get/
Each of the items in the top row is one of the games. Marathon is the first game, Marathon

urandal is the second, and Marathon: Infinity is the third.
From the second row, you need to of course download the Aleph One version for OS X.
You can follow the instructions that come with the downloads, but what you'll basically be doing is replacing the marathon applications with the new aleph one application. Aleph one will then access the marathon files (map data, weapon data, etc.) and allow you to play marathon on new systems.
You can also download the items in the third row (e.g. HiRes Textures and HiRes Landscapes) so that the extremely dated graphics look slightly less dated
You can also look at the games on sourceforge.net
Wish I could help you more. What genre of games are you looking for?
I love games, but I'm really frugal, and I won't pay for a game unless it is REALLY, really good. I generally check reviews on several sites (like ign.com or
http://www.insidemacgames.com/) and if it's getting really high marks across the board, like at least 80/100 on most sites, I'll give it a chance.