Theoretically any Source Game is Multiplayer Capable. So we tried it with Portal, first we got kicked from our Server after a few Seconds. Then we noticed that this not happens when VAC is turned off.
You just need to:
-Enter the Portal Start Parameter -unsecure
In console:
-maxplayers X (to the amount of players you want)
-map (whateveryouwant)
-sv_cheats 1 (in order to get the all Weapons AND the Portalgun)
Every player has his own 2 Portals, and you can shoot weapons through it. Physics also work good. It just crashes when a player get near it, but anyway, it a whole lot of Fun =D.
Proof that it works:


ROCKET PENIS:

ASS MAGNUM:


Portals for everyone:

Infinite Rocket:

More Multiportal:

2 Infinite Rockets:

Multiportaled Grenade:

More Multiportals:
