Portal

The original game of Portal was a computer game available with the purchase of the "orange box" (a box you could buy which had 3 games in it, Portal, Half life, and Team Fortress) originally for the pc and xbox. Portal starts the main character, Chell, off in a small room from which you awaken from a long sleep. You are not aware as to how you got there. A voice projects giving you information on your purpose little by little. In all, you are brought there as a test subject to complete different tests created by a group of scientists in an organization called "Aperature". The head computer gives you a portal gun which fires 2 portals, a orange and a blue one. When you fire both onto a wall, you can go in one and come out the other. You use this device to complete tests that get progressively harder as you go. As you complete more and more tests, you begin to become suspicious as to what the computer has in store for you after she has finished with the testing. You learn you must find a way to defeat the head computer and escape the lab to freedom.
This game was not thought to be as huge of a hit as it was and because of its success, the creators decided to create a sequel.