Drupal 8 status
See https://github.com/agentrickard/domain for the most current work.
The Domain Access project is a suite of modules that provide tools for running a group of affiliated sites from one Drupal installation and a single shared database. The module allows you to share users, content, and configurations across a group of sites such as:
<-- can use any domain string
<-- treats non-standard ports as unique
By default, these sites share all tables in your Drupal installation. The Domain Prefix module (for Drupal 6) allows for selective, dynamic table prefixing for advanced users.
Module installation requires edits to your site's settings.php file. See the online installation guide for details.
The module uses Drupal's Node Access system to determine what content is available on each site in the network. Unlike other multi-domain modules for Drupal, the Domain Access module determines user access based on the active domain that the user is viewing, rather than which group or site the user belongs to. For more information about Node Access in Drupal, see http://api.drupal.org/api/group/node_access/7
This module uses some advanced Drupal concepts and requires that you understand and control your site's DNS configuration. See this case-study or this more recent Row Eleven Wine Co. post for more details about using Domain Access.
You can also watch video of the DrupalCON Paris session "Managing Multiple Sites with Domain Access" (from 2009).
If you are looking for a module to provide subdomains to users and groups and do not need to affiliate content, take a look at Subdomain.