Drupal Q&A site at stackexchange.com
Stackexchange provides a service to communities that allow people to ask questions and receive answers for a particular topic. More and more Open Source communities have created such sites on Stackexchange with good success, even there are some arguments about the fact that the Stackexchange site itself is not licensed under an Open Source licence. However, the data (i.e. the questions and answers) are made available to the public via a common creative licence.
If people prefer a fully Open Source compliant site, Shapado.com is available, whose source code is available under a Open Source compliant licence.
Other successful, open source community sites at Stackexchange are:
The outstanding feature of these Q&A sites is that a lot of very knowledgeable members of the particular community participate at the site and through voting mechanism the questions and answers are kept up at a very high quality.
I hope many in the Drupal community will join this effort and in turn help increase Drupal's popularity since good questions and answers help the usability of drupal and make it easier for people to use and run websites based on Drupal.