Apart from the various (IMHO horrible) php bb forum workalikes, I'm only aware of Discourse:

http://www.discourse.org/

[edit: Personally I'd love to see this end up being a real effort to make a decent web front-end to/for (and an interface for integrating into web sites) mailman3. I'm sick of half-assed email integration for discussion sites (with poor support for non-html mail).

Still, as most news sites have a vested interest in forcing (web) views (for ad imprints) when none are really needed for viewing the content or participating in the discussion -- I doubt we'll get that.]

A good list of comment platforms is here: http://www.elegantthemes.com/blog/resources/using-alternate-...

The two most popular that I've seen are facebook comments and disqus.

Those are two of the biggest reasons I don't comment on many such sites, ever. Of course, I must be one of the last three technical people who has never had a Facebook account, despite having known about them since the days you needed a .EDU address to sign up.

They do seem the most popular. Neither is open source, sadly.

WashPo was using RealTidBits, now I think they're using LiveFyre (who bought RealTidBits).