Alby_da_Wolf on 20 December 2008
I did it with Fallout 2, I wasn't a fan of apocalyptic scenarios, then I stumbled upon it, given with a gaming magazine, and I was amazed. But I didn't play the first chapter, after, the 2nd chapter plot and environment's vastness appealed me more and I don't like to play not following the time line (this cast of mind still prevents me from watching the "new-old" Star War saga chapters 
).
I bought Quake II too having only played the first chapter's demo (and obviously QII itself's demo) before.
Oh, and Ecstatica II, found it already old in a bargain bin, by then the first was stone age and impossible to find.
...
Oh, yes, how could I forget it, Duke Nukem 3D
I don't remember others...
Edit:
Magic Carpet 2
