Idea: Portal Multiplayer
Would it not be confusing for portal gun users?
How could they differenciate between their own blue portal and the blue portal put by another guy?
NO! THE CAKE! NOOOOOOOO !
you put a floating tag by it, or have unique colours for ever portal player
look what there making its not out yet though 
http://portalmultiplayer.com/index.php
Wow that website is a joke...I hope they aren't really planning on doing anything. The only thing thats actual content on there are some icky looking anti-aliased icons on the homepage. Their "video" is just a mod with the exactly Portal code, a different start-up screen and...well...that's it. I don't see any evidence of other people in their game, any rules...anything. That and the horrible music coupled with the Halo 2 logo on his desktop (god how can you use a computer with that there, it would drive me nuts) makes me believe this won't amount to anything.
Haha, wow - that video is about 50% just getting into the game and loading the map (in a freaking window at that). Then you get to watch him play (and die) at 5fps in an official portal map.
What a load.
its always been a load this is nothing new, we started discusing it atleast 100 years ago
portals only been out for about a year 
so no one thinks that this is true?
There would be no sense of choke points or terrain strategy, and any weapons involved would have to be grafted in from some other game.
I would love to see screenshots or a design doc or a tech test of some way to make this an interesting and fun game. But I just don't see it.
I think co-op would be cool, the one with one color for each player. and there could be flings.
I was thinking, what about one where there is only one portal gun, and the 2 players have to "give" the gun to each other by bumping into each other or pressing E near them. this could make for some interesting puzzles where the player without the gun is in another room, but is like a walking companion cube, pressing buttons and stuff. then there could be slots you could pass it through. just a thought.
thats not a bad idea at all. HOWEVER. 2player multiplayer games rarely do well. people want to get on and KILL KILL KILL
now if this was just a mod someone worked out. it might be ok. but i doubt it would get LOTS of acclaim, cuz you would have to find a friend, and since we spend all our time online... we don't have any of those.