Ok I’m not sure if its glitched or a legit offer, but Portal 1 is FREE on steam.
Where there is normally a price, there isnt one so you can get it added to your account for free.
So yeah, this is probably the best free game steam has ever had. Even if you dont want to download it, add it to your library and download it later 
Steam do this often, with different games.
also
FOUND BY YOURS TRULY.
-is modest-
EDIT: Actually, it was brought to my attention by wolv21
PORTAL!!!111!!111!!! THANK YOU!!! lol
I remember they did this a while back while promoting the upcoming portal 2, I got a good 40hrs out of it so def worth the download.
The game is fantastic! I got it the last time it was free. I also played it when it first came out on a friend’s PC.
GET IT WHILE YOU CAN!!
i got the HL2 orange box and downloaded it on steam. now i can play anywhere!