Source: Ferdi Willemse from Border Patrol via mail / PlanetHalfLife

Ferdi Willemse from the mod Border Patrol e-mailed me with the information that there is a Multi Player in Half-Life 2. It's not completely done and several things don't work right, but still it's a start. Since the mail was in Dutch I'll post the essential info on how to get it started here:

Use the following command to get the MP of Half-Life 2:

C:\\Steam\\Steam.exe Steam.exe -applaunch 220 -game MP +map d1_trainstation_04 +maxplayers 12 - console -developer 1 -heapsize 512000 -ip jouw.ip.hier -port 27016 -sv_lan 0

Also HL2World reported the Half-Life 2 Multi Player news, here some words from them:

Here's how:

net_start
sv_lan 0
deathmatch 1
maxplayers (whatever you want)
map (mapname)
restart

It will add you to the master server and it works. I know, it looks like the leak, but I assure you this is the retail HL2.

The full post, including a screen and test map, can be found here. Thanks to Ferdi for pointing this out.