[Mahn]

On a semi-related note, it's silly how there isn't a "canonical" StackOverflow-like site for open discussions yet.

[zevyoura]

Reddit comes close. I don't know that there can really be a 'canonical' discussion site, any more than there can be a canonical Q&A site (Stack Overflow is not that, given the existence of Quora, Yahoo Answers, etc. serving other large niches of Q&A).

[TeeDub]

Slate.co looks like it could evolve into a viable contender in this space. I have a few qualms about the way questions are structured, but the site is pretty and nice enough.

[msutherl]

I use Quora for this mainly, but I'm not sure it's "canonical" yet.

[kriro]

Good old irc?