You have until September the 20th to get something you should have played ages ago.
This isn’t the first time the original Portal was free, so if you really still don’t have Portal and/or have not played it yet you don’t really deserve ti play it. But you’re in luck, as Valve is much more forgiving than I am and is offering one of the best games known to man for free. Again! Do yourself a favor and download it!
If you miss this chance as well, you are officially a horrible person, because this time they’re testing for that.