Valve-ERC Forums is a forum for mapping and modding, focusing on Source and GoldSource issues. The Half-Life 2/Source section is outdated because it moved to a sub-forum of the SteamPowered forums.

This is the official home of Valve.

It might seem strange to add Wikipedia to this directory but it contain some very useful information for Valve’s fans. The sections on the games are very detailed. It is also available in many langauges.