If you are strictly looking for free legal options, Sven Co-op is a standalone game on Steam that allows you to play through the entire Half-Life campaign with friends for free. The Story: Why It's a Masterpiece
For a safe and reliable experience, the official platforms are your best bet. Because the game is older, it is often available for a very low price (sometimes under $1.00 during seasonal sales). download-half-life-1-game-for-pc-full-version
If you want the full story arc, Steam offers a Half-Life 1 Anthology that includes the base game plus the expansions: Opposing Force and Blue Shift . If you are strictly looking for free legal
Fight off alien headcrabs, zombies, and terrifying vortigaunts. If you want the full story arc, Steam
Deal with the HECU (Hazardous Environment Combat Unit), soldiers sent not to rescue you, but to silence all witnesses.