You must check out this new trailer of Valve's Portal, it will blow your mind.

Portal will ship with Half 2 : Epidode 2, and as the title of the game suggests, the portals refer to dimensions in space that transport you to different areas of the ship. If you view the trailer you'll see that the portal gun can blast through walls creating openings that you can then jump through.

Portal is a new type of single player game that changes how players approach, manipulate, and surmise the possibilities in a given environment in a manner similar to how the Gravity Gun changed our approach to how an object may be leveraged in any given situation.

View the trailer at :Game Videos.com Edit : As I understand it's still undetermined as to whether Portal will feature a multiplayer mode.