Hah, I might be the resident "hardcore" gamer here. Unfortunately, I don't do any Mac gaming, or at least not much. I suggest you take a look at some free indie titles - something like
Gamma Bros is available in either Flash or native Mac formats, if you wish.
If you want to experience some of the best storytelling gaming has to offer, I really do suggest you check out Half-Life 2 and the Half-Life 2 Episodes. Unfortunately, they're only available for PC or Xbox 360, but they're readily available (if you download
Steam, you can purchase them online and download them right away). As for the suggestion of F.E.A.R multiplayer, I played it for a while when it went free but didn't enjoy it. For multiplayer, I strongly suggest Team Fortress 2 (also available on Steam) - highly addictive and very beautiful.
I'm sorry that most of these were PC/console only titles - if I can find anything that's good and available for Mac, I'll let you know!