Zork: The Undiscovered Underground, a prequel to Zork: Grand Inquisitor. It was released to encourage people to buy it. Unlike it, though, this game is purely text, extremely short, but interesting none-the-less. Because of how simple it is, it was released free.
I'd rather play Grand Inquisitor, but if you want the free alternative, then I suggest finding and playing this game.
Flame me if you want for some reason, as you know, I'll not be here to witness it unless I feel like it.