Valve has occasionally made the full game free for limited periods, such as during the "Learn with Portals" initiative. While not currently active, these events are worth watching for.
If you already own the game, the high-end graphical overhaul Portal with RTX is a free download that adds modern ray-traced lighting and high-res textures. portal-1-pc-game-free-download
Portal is a groundbreaking first-person puzzle-platformer set in the mysterious . Valve has occasionally made the full game free