The Portal is free again

As part of the promotion, the first part of the Portal can be downloaded free of charge until September 20, after which it will be fully owned by the user.

The game was born under the hands of Valve back in 2007. Since then, he has won no less than 70 awards. 
Portal is a single-player action and puzzle video game. The essence of the game is that the player must use a teleportation weapon ("portal weapon") to solve skill and logic tests. The weapon can create a gateway between two spaces. In 2007, Portal was named one of the most original games of the year, although it received a lot of criticism for its short playing time. 

There are many on YouTube such a video we can find
which shows the gameplay.

The Portal is now open until September 20 for both MAC and desktop PCs free download.

Source: store.steampowered.com