Black mesa, which is a digitally remastered and reworked version of the original Half life is free to download and play.
It's the game Half life should have been. Added dialogue, characters,enemies and modern graphics. What have you got to lose?
You just need to download and install Steam, and their source code to play it.
Seen a few reviews giving it 9.7 out of 10.
I'm certainly enjoying it.
I'ts further on than in the original.
Head back to the main entrance ( the terminal where the guard complains of the system crashing) and find the ventilator shaft blown open. Crowbar is wedged in the door handles.