A perfect time to replay classic games. 

Valve is getting ready to launch Half-Life: Alyx for VR headsets in March 2020, but in the meantime, they're giving away each of the Half-Life games for the next two months. Please note that this is a two-month-long trial, so you can't keep these games. The games are compatible with Windows, Linux, and macOS.

You can now play the original Half-Life (the Source version of the game with added physics), its expansions, Half-Life: Blue Shift and Half-Life: Opposing Force. Then you can also replay Half-Life 2 and its two episodic expansions.

You can find the game right here.