Downloading Portal 13.13.1115.1551 from the developer's website was possible when we last checked. We cannot confirm if there is a free download of this software available. The software lies within Games, more precisely Action.
We recommend checking the downloaded files with any free antivirus. This PC tool can operate with the following extension: ".vst". You can set up this PC software on Windows XP/Vista/7/8/10/11 32 and 64-bit.
The default filenames for the program's installer are portal2.exe, 1CAMP.exe, RUN_GAME.exe, PS3SavePortal.exe or Portl.exe etc. The most popular versions of the Portal are 13.1, 2.0 and 1.5. This program is an intellectual property of Valve Corporation.
From the developer:
Portal ™ is a new single player game from Valve. Set in the mysterious Aperture Science Laboratories, Portal has been called one of the most innovative new games on the horizon and will offer gamers hours of unique gameplay.
You may want to check out more software, such as DAoC Portal, Portal for ArcGIS or Varian Portal Dosimetry, which might be related to Portal.