IGN takes a closer look at Portal.
"The game itself is looking like the next evolution in puzzle games. You thought 3D Tetris was mind-blowing, wait until you see Portal. For those who haven't been following Portal, the concept is fairly simple. You have a modified version of the grav gun. It can lift items, but can't shoot them forward. That's okay, because this new gun has a much more useful feature. Aim and shoot at any area (with exception of resistant black-padded walls) to create an entrance and then an exit portal. You can move through these portals or send other objects."