Is there a community driven unified article database?
Sadly, not yet...

Many publishers view their bibliographic metadata as commercial property and hold-on to it fairly tightly.

Mendeley will release little bits of the article data they have accumulated from all their users, but they will not let anyone download all of it - it's valuable property that they want to keep 'in-house'.

There are a few truly community-driven projects trying to change this e.g. Open Citations &amp; Open Citagora.

These projects aim to build truly public &amp; open databases for anyone to build upon &amp; annotate. The other tools mentioned in the question aren't really public or fully open.