The first chapter of Half-Life 2, remade in Half-Life 1's engine. To install simply extract to your Half-Life folder.
A preview of Half-Life 2's first chapter remade in Goldsrc. Please note this version is INCOMPLETE, and some things may be broken and/or incomplete.
This works like any other Half-Life mod, so just extract the HL2ClassicDemo folder to your Half-Life directory. Then relaunch steam and it should be in your games library.