If you know of a good site that's not listed here, please use the contact form to tell me.
Andrej Mernik Andrej has kindly mirrored most, if not all, of my Half-Life 1 files.
ValveTime As close to the official Valve news site as you could get. Hosted by Valve themselves, this site covers all the games from Valve and the news. Very active forums and lots of content.
Lambda Generation LG posts news about Valve and its community. It is updated regularly and has some great articles.
ModDB This site changed the modding community and few sites can claim that. In addition to having the largest mod database, it also has lots of feature articles and interesting content. The forums are very active too.
Combine OverWiki Without doubt the best site for detailed information about all the Half-Life games. It is an immense resource and every fan should visit it and learn more about the games they love.
SnarkPit Snarkpit is another site that seems to have been around for a while and has gone through a number of design changes. It covers mostly maps for some of Valve games and allows registered users to submit their work.
TWHL The Whole Half Life is a site that has been around for a long time. It has a very active community and contains lots of maps. It covers a few different games and has pretty active forums.
Interlopers Interlopers covers Valve news and has a very active community. It has a lot of modders and mappers as its members and the feedback given for WIP is impressive. There is a portfolio section that is very useful. All in all a great site.
Radiator Blog Website of Robert Yang, well-known modder, best known for his Radiator series of Alt mods.
HL Story Saga An unofficial guide to the detailed story behind the Half-Life games. Interesting reading, but remember, it is not official.
FPS Level Design Blog "This blog contains lessons learned by me through the process of mapping for Single Player games, primarily, Half Life 2, Portal and Left 4 Dead. The focus of this blog is on the player experience.
ModMatic Hosted by Adam Foster (developer of Minvera), this is a detailed review site that does not post very often.
Amphibian Mods This site provides working links to quality mods and maps for a selection of first person shooter games.
P Comos A simple site with a great list of Half-Life 1 and Opposing Force mods.
Planet Half-Life In some ways the grand-daddy of Half-Life sites. It is not as good as it used to be but it can still be useful, sometimes.
3D Buzz Tutorial site that covers modding, coding, graphic arts etc. Hugely popular and very useful.
Map Factory Map-Factory is a database hosting custom maps for any FPS. If you have made a map or simply came across a good map, feel free to submit it!
DeadEndThrills specialist game-screenshot community. It has some amazing shots - very artistic.
Metrocop This website is a blog, created by Mythos, for comics created using Garrys Mod.
Valve Community Valve Community is a huge database of websites that cover Valve games and its community. Registered users can add their own site and update its details. There is a multi-screenshot feature and rating system. Sites are listed by game and content tags.
Podcast17 Podcast17 is a weekly podcast about the Valve gaming community. It features news about Valve, community news, including map and mod releases/reviews, interviews and occasional competitions. Highly recommended.
TheSteamCast Steamcast is a fan-made podcast that hosts discussions about Valve and Steam, but also a wide range of gaming related subjects such as video game digital distribution, the art and design of video games and other relevant news and topics.
ThePodKast The podkast network is a group of shows, focused around the world of gaming. Each of the Podkasts brings you the most up-to-date news, content and discussion on their topic of choice.