Mod DB is the largest website dedicated to supporting independent development of games

Modifikace is a Czech news and review site for single player Half-Life 1.

Mods Online is a news and community forum, with a very active user base.

My Aperture Labs: Welcome to the Aperture Science Enrichment Center’s Online Community Networking Web Service (or ASECOCNWS for short). Here at the ASECOCNWS, test subjects may attempt to solve Portal puzzles, build their own Portal puzzle environments, or simply talk about cake.

Nana’s Paradox covers Source games and runs articles and news about maps and mods.

Netday is dedicated to custom maps of high quality only. Maps are added regularly (at least one per week). It also covers mods like Dystopia, Portal Prelude and JailBreak. The map reviews are both in English and in French. It is advert free and features direct download to all files.

OzSource is the home of the Australian Gaming Combine. Steam discussion, Community Portal, Gaming news and opinions, critiques, retrospectives. Oz Source has been resurrected from its Half Life 2 Deathmatch origins.

Pájaros de Barro is a Spanish Gaming Community which offers public servers and support (tutorials, videos, forums, articles…) for Team Fortress 2 and Left 4 Dead as well as updated and fully spanish translated news and interviews related to Steam and Source Engine based games.

Phenomena On Break is a community brought together by their love of Valve games and games in general. They have used Valve games for over a year to make over one hundred comics based in the Half-Life universe, as well as the use the Source Engine and Gmod to create original universes and characters. They [...]

Planet Half-Life is the daddy of them all. It’s been online longer than most and is generally updated daily.

PlanetPhillip.com contains the most comprehensive database of single player maps and mods for the Half-Life series of game on the Internet.

SLUM is Czech Steam community and fan portal about Steam and Valve games.