Add Steam forums to those. 9 out of ten times, game walkthroughs or hints can only be found on Steam, according to whichever search engine. But only if you have a login there, of course...
Indeed, I just checked, it's not the entire Steam community site. Looks like it's a per-subcommunity (per-game?) setting whether the community section is open to the public or not. Luckily there's a lot of content that's not behind a login wall.
