Feel free to use an example. I am thinking things like guides, tools that help a player make certain in-game choices, discussion boards, services that enhance player interaction, mods, blog/news, etc. I am sure sites like this exist for really popular games like WoW. I am wondering what the community here thinks makes for a successful game-specific community website.
Above all, I think it's the community. The community's demeanor is often not in the developer's control but there are things that can be done to promote a friendly culture. For example, rewarding players for helping newbies, a decisive ban every now and then, and giving players the power to reward other players for being helpful.
Next up is giving players the ability to feel proud about their game achievements. This can be something like posting replays for other people to analyze, hosting elite tournaments, talking about strategies, etc.. Mostly the standard forum stuff.
Finally, there's the developers themselves who must communicate with the community and make them feel like their effort and skills contribute to the game's development. Glance over every now and then, respond to some suggestions, and let them know if a suggestion was implemented.