The Trenches: Fields Of Horror is a World War One modification for Half-Life 2.

The Whole Half-Life AKA TWHL is first and foremost a community of mappers and modders. It has very active forums and plenty of tutorials.

Thinking With Portals is a website dedicated to Portal.

TotalHL.hu is a Hungarian Half-Life portal.

UberCharged.Net contains random crap referring to TF2, hopefully vaguely entertaining.

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 The Half-Life 2/Source section is outdated because it moved to a sub-forum of the SteamPowered forums.

World Of Level Design is a website/blog aimed at those interested in Environment Art and Game Level Design. It contains plenty of original content, including tutorials, articles and interviews. There is also a lively forum.

Zombie Panic: Source is a survival horror MP HL2 modification. Play as either the survivors or the undead, trying to wipe each other out or completing objectives to escape the horror.