Portal is awesome, I <3 my Weighted Companion Cube.
It is really quick and easy to play as long as you aren't drunk. Seriously though it has a very interesting gameplay and allows for a great interaction with the environment.
The requirements say 1.7ghz, would this run on my 1.5ghz dual core? (I played it on a friend's computer)
Oh and by the way: Digipen is a college for video game design, the people behind the game graduated from there.