Besides LW, what are some other online communities with very high signal/noise ratios?

Some good indicators: Lots of original content, meaningful and well-presented information, respectful conversations with a high level of discourse, low levels of trolling/strong community norms, very strong domain-specific knowledge, people know each other by username.

Another heuristic: somewhere where you would not want to share the link with a lot of people lest the quality be diluted with newcomers. (Hopefully you consider LW a strong enough pool to draw from).

Examples I can think of:

• The Straight Dope Message Boards is a great general topic forum with low rates of trolling.

• <strike>The Oil Drum, is a strong web community specific to the Energy industry.</strike> Stopped updated this past September (thanks knb, I hadn't visited for a couple months).

• Niche subreddits are often a great resource, so much so that when I'm looking for information I often do a reddit search before looking over the greater internet.

Things that wouldn't count:

• Social news sites, like hacker news or the the large subreddits, which tend to have a lot of noise.

• Gwern's Google+ feed has perhaps the single highest signal/noise ratio of anywhere I've seen on the web, but obviously is just the one person.

• The Erowid Experience Vaults is a solid collection of crowdourced drug experiences, but doesn't have a community attached.

Any others?