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.

NEOTOKYO is a first person shooter that provides a visceral & realistic combat experience in a rich futuristic setting. It is greatly influenced by our admiration for Masamune Shirow (GHOST IN THE SHELL) and Katsuhiro Otomo (AKIRA), and has been in production since October 2004.

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.

Obsidian Conflict is a co-operative modification created for Half-Life 2. It is a community based mod with focus on fun, co-operative and physics-related puzzles. The mod has an open story, but is still set in the Half-Life 2 universe.

OnlineGame.se is a Scandinavian site that covers Team Fortress 2.

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.

Planet Fortress is IGN’s portal for all things TF. It is more of a reference site than a regularly updated center.

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