Now with a Steam page that doesn't expressly list Mac as a supported platform, we understand your skepticism regarding the headline to this very post. Indeed Half-Life has been ported to both OS X and Linux today.
We tried it on OS X and it totally worked, though we've yet to try it on Linux. Odds are if you have a Steam account, you probably already own Half-Life – but if for some reason you need to purchase this iconic shooter, it'll set you back $9.99 on Steam right now.