Well here's some interesting timing. Just days after I finished playing Portal for the very first time (more on that in a bit), Steam is offering the game absolutely free.
Specifically, from now until September 20, you can download Portal (PC and Mac) absolutely free. Yesterday, it would have cost you $10.
Update: Many readers have reported difficulty signing up for a Steam account. My only suggestion is to keep trying, as I'm sure Steam's servers are overloaded. I'll update further if I learn anything more.
Update #2: Steam tech support just tweeted this fix: "… Read more