So we talked more or less centering around PC on Steam, right? I'd recommend Don't Starve or Half Life 2, both games can be played on most machines I would think.
For Don't Starve I think it could bring about great discussion on strategy to last even more than a day and HL2 is just great. Lastly, I'm super stoked this was put together.
I'm also voting for Half-Life 2, I feel this is a game everyone should play at some point, and I know personally I have no problem replaying it.
I'm going to officially vote Half Life 2 as well. Great pace, cheap ($5.00 on sale on Steam right now), on every single OS, and a perfect intro for people who haven't played FPS games before, in my opinion. Let me know if you want to play but don't have a copy, if HL2 ends up getting picked! Thanks, I hope it's fun for everyone!
I'll vote for Half-Life 2 as well. It's available on pretty much any PC now, plus PS3, XBox 360, and XBox Classic (which is the version I'll be playing if it works on 360.) If you're buying HL2 now, you might want to wait until Sunday, when the deals roll over, it might go as a Community Choice/Flash Sale for $2.50 between now and then (if $2.50 is worth waiting for to you.)