I heard Ultima IV was the game that changed the face of RPGs. It's available for free when you sign up for GOG.
Frog Fractions may not be a true text based game, but it has text based section. I tried when I heard about it on Giantbomb and it is hilarious. You can find that for free here: Frog Fractions | Twinbeard Studios
It’s way too long, haphazardly plotted and teases on being more than a joke machine without really following-through, but I laughed more at Anchorman 2 than almost any other comedy this year.