It has just been given its multiplayer part today.
Half Life 2 Deathmatch has just been released and it is pretty good fun. Use the gravity gun to pick up stuff like radiators, toilets and explosive barrels and shoot them at people. It's great fun...
"Mrs. Jones, I'm sorry to inform you, but we've run the tests, and it appears that you have XP. Now don't cry - it's bad, but it's not a death sentence. Modern science has advanced in recent years, and it's now possible to live a reasonably happy life with XP. And there's a survivor's group that you'll want to meet as well."