I think that true Joomla fans have heard about this project - Joomla Multisite Support. It is not a secret that we are big fans of Joomla and we are waiting  impatiently for the official release of this wonderful project. In a few words this project will open great opportunities for all Joomla community.

According the information from Joomla!.org main advantages of the new project are:
- A simple interface to install multi-site configuration.

- One code base w/one database

- System is compatible will all existing modules/components/plugins

- Will enable more large organizations to adopt Joomla

- Share users, session, modules, components and content between sites

- Enables full control over Joomla URLs, grants ability to make your URLs shorter, user friendly and search engine friendly.

More detailed information about this project you can find here.