While I've obtained Portal for free via Steam (go to this store.steampowered.com/freepor…
if you also want Portal for free, but the offer will expire after 24th May), I later on took my opportunity to activate my legit copy of Half-Life that I got for free.
And when I activated the game, the Steam application told me that I've activated the Platinum Pack version of Half-Life, thus I recieved a bunch of old Valve games (including Counter-Strike 1.6, the Half-Life expansions and Team Fortress Classic).
Sweet! More free games.
And to those who wondered how I got Half-Life for free. Well, about a year ago, some of the staffs at the college I studied brought some boxes filled with game posters (mostly WoW), Warcraft III strategy books and legit Half-Life copies. And everyone were free to take them.