An intrepid Redditor going by jackaljayzer was lucky enough to get a Half-Life preview disc, intended for journalists at the time, by a friend in Bellevue, Washington (where Valve is based). The disc contains screenshots, trailers and a demo build of the Half-Life 1997 alpha.

Jackaljayzer has put the contents of the disc up on the web and is auctioning the disc on eBay. You can download the alpha right now from this link. To play the alpha delete or renamed the "...\Half-Life 0.52\Half-Life\Opengl32.dll" file and then run "...\Half-Life 0.52\Half-Life\enginegl.exe".