: During the Half-Life 25th Anniversary celebrations, Valve briefly made the original titles free to claim. It is worth keeping an eye on Steam News for similar future promotions.
: For a modern take, the fan-made project Black Mesa: Blue Shift (available on the Steam Workshop) aims to remake the expansion within the Black Mesa engine. You must own the base game Black Mesa to play it, but the mod itself is free. Why Avoid Unofficial Downloads? Half-life Blue Shift Free Download
: This is the primary official source. While not permanently free, Valve frequently includes it in deep-discount sales (often for less than a dollar) or as part of the Half-Life Complete Bundle . : During the Half-Life 25th Anniversary celebrations, Valve
: Unofficial versions often strip out the High Definition models or the essential soundtrack. You must own the base game Black Mesa
Searching for "Free Download" executables from third-party sites often leads to:
: Many "repack" or "crack" sites bundle malware or adware with the game files.