There's a lengthy update on the development of Half-Life 2 co-op play on the Sven Co-op page. It is mentioned that co-op support for the Half-Life 2 single-player campaign is their top priority, but ...

_However_ you have to have the original Half-Life installed (from the CD, not Steam) so that Sven-Coop can access the level data. If you only have the Steam version installed, you will only be able to ...

Long ago, before Left 4 Dead had us merrily killing zombies with one another, there was a popular third-party Half-Life mod that was on the cutting edge of co-operative game play for first-person ...