When Half-Life 2 was first released (before hl2dm) many people found ways to play multiplayer. At first it was very buggy (VALVe's fault), but it got better, little by little. The projects have since been abandoned, however one may find the mods still floating around on the interwebs. I was thinking, I could possibly modify the modified files to make Portal multiplayer compatible. It shouldn't be very hard, and though I should be doing homework, hopefully I can have it done by later tonight, or at least a beta version of it. No guarantees that it will have a server list like other games, but you can use the "connect" cvar to do it.
I however am unable to run a server because my router is fucked up, so if anyone would like to test, please reply. I can join however. The nice thing about portal though is that there are animations and player models, and hl2 has been around long enough that many bugs were fixed. And because Portal is hl2, you can use hl2 weapons in it as well. My only concern will be the fact that the game was only made with one portal gun so I am wondering if you will be limited to a maximum of 2 portals (one yellow/orange and the other blue).
Peace out till I am "finished with homework" (which I should be doing yet haven't even started.
PS- when I say later tonight, that probably means it won't be until my parents go to sleep that I start working on it. Being in the room next to them doesn't quite help the matter... 