Portal is based off a game called "Narbacular Drop" (which is a short puzzle game by "Nuclear Monkey Software" (Good name right?) ).

Source: en.wikipedia.org/wiki/Narbacular_Drop

Also heres a vidoe of some one playing the game (not my video), you can see how Value have used some bits for Portal and stuff. - http://www.youtube.com/watch?v=tNG52b_jLYc

Zoey from Left 4 Dead was originally going to be the protagonist for Portal, however due to later model updates she was cast a survivor for Left 4 Dead and Chell became the new protagonist.
Both also have a few similarities in their design

(07-29-2012 04:18 AM)BumblebeeCody Wrote:  Zoey from Left 4 Dead was originally going to be the protagonist for Portal, however due to later model updates she was cast a survivor for Left 4 Dead and Chell became the new protagonist.
Both also have a few similarities in their design

The Portal 2 version of Chell does look abit like Zoey I sopose if you aged her the Potal 1 version of Chell would as well, and they all have the same stlye of hair, I agree with that yer.